Raiders of the Lost Ark (1981)

1

Director - Steven Spielberg

While chasing Marion Ravenwood (Karen Allen) after she's been kidnapped, archaeologist and adventurer Dr. Indiana Jones (Harrison Ford) runs into a large sword-wielding bad guy dressed all in black. Instead of fighting him in what would surely be a losing whip versus sword battle, Indy simply pulls out his revolver, puts the man down with one shot and moves on.
The original script called for a long sword fight but a day earlier Ford got a severe case of food poisoning and didn't have the energy to film the scene as written. After a discussion with director Steven Spielberg, the scene was changed and became an iconic part of Indiana Jones mythos.

Zoolander (2001)

2

Director - Ben Stiller

In this scene involving former hand model J.P. Prewitt (David Duchovny) and the dimwitted male model Derek Zoolander (Ben Stiller), Prewitt - a conspiracy theorist - explains how the fashion industry has been behind every high profile political assassination of the last hundred years.
Zoolander asks, "Why male models?" Prewitt answers with a lengthy explanation, after which Zoolander responds again, "Why male models?" Stiller forgot his original line and just repeated his previous line instead. This prompted Duchovny to ad-lib his response "Are you kidding? I just told you like a minute ago."
The scene ends up reinforcing the movie's narrative of the brainless male model stereotype and Stiller turned a gaffe into one of the funniest parts of the film.

The Godfather (1972)

3

Director - Francis Ford Coppola

Vito Corleone (Marlon Brando) is more than the cold-hearted head of a powerful Italian mob family. That trait shows when he sentences a man to be beaten as retaliation for the beating of another man's daughter - all while gently stroking a cat.
Thing is, the cat was never part of the original script. Some reports say that Coppola plopped the feline into Brando's lap just before filming began. Other reports say Brando found "il gatto" roaming around the set, picked him and gave him an offer he couldn't refuse (heh).

The Fugitive (1993)

4

Director - Andrew Davis

In this famous showdown between Richard Kimble (Harrison Ford) - a doctor wrongly accused of murdering his wife - and U.S. Marshal Samuel Gerard (Tommy Lee Jones), Kimble gets the jump on Gerard in the sewers. Instead of shooting the Marshal and making things worse, Kimble pleads his case to him saying, "I didn't kill my wife!" Gerard, with a sober tone and intense look on his face, responds with a simple, but brilliant and ad-libbed, piece of dialog, "I don't care."
The line wasn't part of the script but those three words reinforced to Kimble, and audiences, that it didn't matter to Gerard whether the doctor was guilty or innocent of the crimes for which he was accused. He was going to get his man - no matter what.

The Dark Knight (2008)

5

Director - Christopher Nolan

As the Joker (Heath Ledger) waits quietly alone in jail after having been arrested by Jim Gordon (Gary Oldman), Mayor Garcia (Nestor Carbonell) shows up to look over Gotham's latest scourge. While there he also promotes Gordon to the position of Commissioner.
As the officers in the room applaud the announcement Ledger begins, unscripted, to slowly clap - never changing his facial expression. It was just a simple improvisation but one that was unsettling and darkly brilliant.

RoboCop (1987)

6

Director - Paul Verhoeven

When antagonist Clarence Boddicker (Kurtwood Smith) is taken to the police precinct after receiving a solid whooping by RoboCop (Peter Weller), Boddicker spits a bloody glob onto the paperwork of the desk sergeant, followed by the line "Give me my f*ckin' phone call!"
Smith and Verhoeven briefly had discussed the unscripted moment before filming the scene but neglected to inform the extras - which was evident by their real and disgusted surprise as the scene unfolded.

Pretty Woman (1990)

7

Director - Garry Marshall

In what became one of the most famous scenes from the film, Edward Lewis (Richard Gere) presents call girl Vivian Ward (Julia Roberts) with a gorgeous and rather expensive diamond necklace. As Roberts reaches out to touch the precious jewels, Gere - in an unscripted playful moment - quickly snaps the box shut genuinely surprising her.
Her laugh was so honest, and the scene so good, that Marshall decided to leave it in the film as is.

Being John Malkovich (1999)

8

Director - Spike Jonze

Shortly after John Malkovich meets Craig Schwartz (John Cusack) on the side of the road he wanders off mad. As he does, a car passes by with a man leaning out the window. The man throws a can hitting Malkovich square in the back of the head while yelling, "Hey Malkovich! Think Fast!" - causing the Oscar nominated actor to scream out in legitimate pain.
Neither the can throwing nor the reaction were scripted but the drunken extra in the car felt the opportunity was too good to pass up. Jonze thought the scene added to the character's frustration and left it in.
Instead of being fired, the extra was added to the final cut of the film and given a raise.

Caddyshack (1980)

9

Director - Harold Ramis

One of the best and most quoted scenes from this film is "The Cinderella Story" where groundskeeper Carl Spackler (Bill Murray) mutters a story to  himself about an unknown golfer winning The Masters.
This entire scene was developed by Murray on the spot saying in his 1999 book Cinderella Story: My Life in Golf: "The Cinderella Story was a spur-of-the-moment idea. 'Get me some flowers,' I said. 'Four rows of mums."

Dumb and Dumber (1994)

10

Director - Farrelly Brothers

There are many scenes in the film that show how moronic and simple-minded best friends Lloyd Christmas (Jim Carrey) and Harry Dunne (Jeff Daniels) can be but this one showed how annoying they can be on car trips - and it was entirely unscripted.
Even hitman Joe Mentalino's (Mike Starr) hissy fit reaction to the scene was unscripted, which makes the scene that much funnier.

Knocked Up (2007)

11

Director - Judd Apatow

Crafting a good and funny insult is one of the hardest things to do but Seth Rogen and Paul Rudd are two of the best - proving it in this scene of put down jokes.
This entire exchange between Pete (Rudd) and Ben (Rogen) while in the car was completely ad libbed by the two actors. The scene is only a few seconds long on the final cut but as an extra on the DVD, the scene goes on for over six minutes.

Good Will Hunting (1997)

12

Director - Gus Van Sant

In this scene between therapist Sean Maguire (Robin Williams) and math genius Will Hunting (Matt Damon), Williams proves that comedic-minded actors usually give the best ad libbed scenes.
The entire story about Maguire's flatulent spouse was made up on the spot by Williams and not a part of the original script.

The Dark Knight (2008)

13

Director - Christopher Nolan

Originally, the Joker (Heath Ledger) was supposed to walk down the street while the explosion at the hospital began, get on the school bus during the scripted pause, and the bus would drive away while the explosion finished.
However, Ledger stopped walking during the pause and in a moment of improvisation began fidgeting with the remote detonator in a very Joker-esque manner - bringing a slight amount of dark humor to what would have just been a serious scene.

Aliens (1986)

14

Director - James Cameron

Chaos and confusion are everywhere after the first attack by the xenomorphs decimate the Space Marines and their drop ship crashes. As the crew tries to get their bearing and fully understand what just happened, Private Hudson (Bill Paxton) - ever the pessimist - laments "That's it man, game over man, game over! What are we going to do now?"
The original line didn't include the "game over" part and was ad libbed by Paxton.

Tootsie (1982)

15

Director - Sydney Pollack

During this scene, aspiring playwright Jeff Slater (Bill Murray) was required to appear to be talking throughout the entire party; however, there was no dialog written for the character.
As a natural entertainer and comedian, Murray improvised the entire scene.

The Usual Suspects (1995)

16

Director - Bryan Singer

Christopher McQuarrie wrote only one line for this scene -"Give me the keys, you f*cking c*cksucker!" - it was up to the individual actors to deliver it however they wanted. McQuarrie actually plays the cop speaking with the suspects and both his line to Fred Fenster (Benicio Del Toro) "In English please?" and Del Toro's reaction were unscripted.
According to interviews on the DVD, the laughing during Del Toro's delivery was due to his constant farting while filming - boys will be boys.

The Warriors (1979)

17

Director - Walter Hil

lIn this scene, the script called for Luther (David Patrick Kelly) - leader of the vicious New York gang the Rogues - to drive up and provoke rival gang The Warriors to a fight in the streets by clinking bottles together.
Kelly spontaneously added the now famous line "Warriors, come out to play!”

The Godfather (1972)

18

Director - Francis Ford Coppala

Corleone family capo Peter Clemenza (Richard Castellano) orders his henchman Rocco Lampone (Tom Rosqui) to carry out a hit on Paulie Gatto (John Martino) for his betrayal of Don Vito Corleone (Marlon Brando).
Castellano's original line was "Leave the gun" but drawing from an earlier scene where Clemenza's wife reminds him to bring home some cannoli, he improvised the now famous line"Take the cannoli."

Dr. Strangelove or: How I Learned to Stop Worrying and Love the Bomb (1964)

19

Director - Stanley Kubrick

Nuclear scientist Dr. Merkwürdigliebe or Strangelove (Peter Sellers) was confined to a wheelchair for the entire film - but Sellers decided to spontaneously stand at the very end of the film, take a couple of steps and proclaim, "Mein Führer! I can walk!"
In a process known as "retroscripting", Kubrick changed much of the script he co-wrote with Terry Southern to incorporate much of Sellers' improvised dialog, including this now famously unscripted scene from the end of his black satirical comedy.

Saving Private Ryan (1998)

20

Director - Steven Spielberg

During a brief break from fighting, Captain Miller (Tom Hanks) sits with Private Ryan (Matt Damon) swapping stories about what it was like back home for them both. The story Damon tells about his brothers and the barn was made up entirely by him during filming.
None of the story was part of the original script.

Jaws (1975)

21

Director - Steven Spielberg

While chumming the waters in an attempt to lure the deadly great white shark within range, Police Chief Brody (Roy Scheider) gets his first look at exactly how massive the killer shark truly is.
Stunned, startled and filled with fear he stands up and utters the now famous line to Orca Captain Quint (Robert Shaw) completely off-script, "You're going to need a bigger boat.”
Turns out, he was right.

Star Wars Episode V: The Empire Strikes Back (1980)

22

Director - Irvin Kershner

As smuggler-turned-hero Han Solo (Harrison Ford) is about to be encased in carbonite, Princess Leia (Carrie Fisher) reveals her love for him. The script called for Leia to say "I love you" to which Solo was supposed to respond with "I love you too".
Ford decided that Solo wouldn't say something like that and instead, changed the line to simply "I know."

Reservoir Dogs (1992)

23

Director - Quentin Tarantino

The script for Tarantino's violent, freshman project called for jewel thief  Mr. Blonde (Michael Madsen) to torture Officer Nash (Kirk Baltz) by cutting off his ear with a straight razor - however, Tarantino didn't give Madsen any specific direction what to do once the gruesome deed had been done.
All of Madsen's lines and actions with the ear were improvised by him.

Casablanca (1942)

24

Director - Michael Curtiz

The scene of Rick Blaine (Humphrey Bogart) putting Ilsa Lund (Ingrid Bergman) and Victor Laszlo (Paul Henreid) on a plane bound for America with the help of Captain Louis Renault (Claude Rains) is chock full of memorable lines but the line listed as 5th in AFI's 100 Years...100 Movie Quotes wasn't even part of the original script.
According to reports, Bogart said the phrase "Here's looking at you kid" multiple times to Bergman while teaching her to play poker between takes.

Annie Hall (1977)

25

Director - Woody Allen

Neurotic Jewish comedian Alvy Singer (Woody Allen) is at a party when his friend passes him a small tin filled with cocaine. As Alvy takes the tin in his hands he has a violent sneeze - sending white powder everywhere. The surrounding actors’ uncontrollable laughter was spontaneous and genuine and Allen decided to leave it in the final cut of the film after it tested well with audiences.
So one of the most famous sneezes in cinema history was never actually intended to be part of the final film - it actually occurred during a scene rehearsal.

The Shining (1980)

26

Director - Stanley Kubrick

Wendy Torrance (Shelley Duval) and her son Danny (Danny Lloyd) hide from the deranged novelist Jack Torrance (Jack Nicholson) in a hotel bathroom. As Jack begins chopping through the door with a fire axe and sticks his face into the splintered opening, he utters a phrase previously made popular by Ed McMahon on The Tonight Show starring Johnny Carson - "Here's Johnny!"
The line was not part of Kubrick's original screenplay and was improvised by Nicholson.

Blade Runner (1982)

27

Director - Ridley Scott

As ex-blade runner Rick Deckard (Harrison Ford) attempts to "retire" the replicant known as Roy Batty (Rutger Hauer), he finds himself in a precarious position. The battle worn replicant shows mercy on Deckard rescuing him from the edge of the building - only to "die" shortly after giving a moving monologue.
As he reminisces about his past he says, "All those moments will be lost in time...," but then Hauer adds the unscripted and philosophical phrase "...like tears in rain."

Midnight Cowboy (1969)

28

Director - John Schlesinger

As want-to-be gigolo Joe Buck (Jon Voight) and crippled scam artist Ratso (Dustin Hoffman) cross a street in New York City, a REAL NYC taxi cab driver who ignored all the "Street Closed for Filming" signs drives through the scene.
Obviously this wasn't scripted and Hoffman's response and actions were all improvised, in character, as a result.

A Clockwork Orange (1971)

29

Director - Stanley Kubrick

Alex (Malcolm McDowell) breaks into a happy song as he and his "droogs" perform a bit of "ultra-violence" and rape. Reportedly Kubrick filmed this scene several times and wasn't happy with it each time - until he told McDowell to just "do anything he wanted".
McDowell decided to belt out "Singing in the Rain" and Kubrick was so pleased with how much better the scene became that he acquired the rights to use the song immediately.

Taxi Driver (1976)

30

Director - Martin Scorsese

When screenwriter Paul Schrader wrote this scene it simply said "Travis talks to himself in the mirror" - there was no specific dialog given. Everything that insomnia-plagued taxi driver Travis Bickle (Robert De Niro) says during his faux-conversation was improvised by De Niro on the spot.
To this day, whenever someone walks by a mirror they can't help but utter his now famous line "You talking to me?"

The Silence of the Lambs (1991)

31

Director - Jonathan Demme

The famous "hssssss" sound made by Dr. Hannibal Lecter (Anthony Hopkins) during his story about eating a liver with "fava beans and a nice Chianti" to FBI agent Clarice Starling (Jodie Foster) wasn't in the original script.
Apparently it was something Hopkins did during rehearsals to creep out Foster - and Demme decided leaving it in was the best way to creep out his audience too.

Full Metal Jacket (1987)

32

Director - Stanley Kubrick

Originally, R. Lee Ermey wasn't even cast in the role as Gunnery Sergeant Hartman but after Ermey submitted a tape of himself spewing insults at group of Royal Marines for 15 minutes straight, Kubrick cast him immediately
Ermey wrote 150 pages of insults and Kubrick estimated that 50% of the character’s dialog was improvised by the former drill instructor.