Came across this on Imgur. Obviously can't post them all, but here's the link:
You're Gonna Need a Bigger Boat
Jaws (1975) 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.
Here's Looking at You Kid
Casablanca (1942) 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) 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) 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.
Singing in the Rain
A Clockwork Orange (1971) Director - Stanley Kubrick Alex (Malcolm McDowell) breaks into a happy song as he and his "droogs" perform a bit of "ultra-violence" and ****. 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.
The Dark Knight (2008) 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.
The Silence of the Lambs (1991) 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) 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.