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.