First, on Stranger Things, part of Murray's conversation with Jonathan and Nancy at breakfast was improvised — you can even spot Natalia Dyer trying not to laugh.
On Euphoria, the flashback that showed Rue fighting with her mom was completely improvised by Zendaya and Nika King — the script simply said "Rue and her mom have a fight."
On Brooklyn Nine-Nine, Captain Holt's iconic "hot damn" line was improvised by Andre Braugher.
On Schitt's Creek, Moira's iconic pronunciation of "bébé" was created by Catherine O'Hara while filming the scene for the first time.
She explained, "I said 'bébé' as a joke or a mistake the first time. Once I hit on 'bébé' and got a laugh from the crew, that was it."
On High School Musical: The Musical: The Series, Joshua Bassett actually came up with Ricky's monologue about loving Nini on the spot.
Joshua included memories from his own relationships and memories specific to him and Olivia Rodrigo's friendship.
On Supernatural, Jensen Ackles spontaneously screamed Dean's classic "son of a [email protected]#ch" line at the end of this scene — you can spot Jared Padalecki laughing in the final take.
On Parks and Rec, all the actors came up with their "drunk off Snake Juice" reactions on the spot — in fact, Amy Poehler was directing this scene and they were trying to make her laugh.
On Sex Education, Aimee Lou Wood hilariously improvised Aimee's "my ham" line after her sandwich was batted away.
On New Girl, Max Greenfield improvised Schmidt's description of his dream girl during this flashback from the series finale.
Jason Segel didn't know Marshall's dad was going to die on How I Met Your Mother until they filmed the scene, so the final cut featured his genuine reaction.
On The Haunting of Hill House, when Theo and Shirley were startled by Nell's ghost in the car, the scene consists of Kate Siegel and Elizabeth Reaser's genuine reactions — Victoria Pedretti made her entrance sooner than they thought.
Creator Mike Flanagan explained, "I just told Victoria before we read the first take to come in substantially earlier than the written cue. The girls had another half page of dialogue to go through as written."
On Parenthood, Craig T. Nelson and Mae Whitman improvised the entire moment when Zeek took Amber to see her wrecked car after her horrific car crash.
On Riverdale, the moment when Kevin kissed Archie while performing "Tear Me Down" from Hedwig and the Angry Inch was unscripted — it was something Casey Cott and KJ Apa came up with while filming.
The moment is a nod to a moment in the original Hedwig and the Angry Inch musical.
On One Tree Hill, during the series finale, Brooke's speech about what made high school so special was unscripted — Sophia Bush simply reminisced about her time on the show and playing Brooke.
On One Day At a Time, the moment when Lydia held Syd's hand after Elena explained that they were harassed after a date was improvised by Rita Moreno.
During The Vampire Diaries Season 5 finale, Ian Somerhalder added in the heartbreaking "baby" when Damon said goodbye to Elena before the Other Side collapsed.
On Veep, while improv was used a lot during preproduction, one moment that happened while filming was when Gary and Mike broke an expensive tea set in London.
On This Is Us, when Randall and William went to Memphis and visited a barber shop, their conversation was unscripted — Sterling K. Brown and Ron Cephas Jones simply talked about their real first haircuts.
On Dead to Me, Linda Cardellini came up with the heartbreaking line Judy uttered after learning Abe died in the moment.
And finally, on The Defenders, Krysten Ritter improvised this hilarious Jessica line when she saw Daredevil's outfit for the first time.