24. Caddyshack - The Cinderella Story
One of the best and most quoted scenes from Caddyshack 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."
23. Dumb & Dumber - The Most Annoying Sound
There are many scenes in Dumb & Dumber 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.
22. Knocked Up - Driving to Vegas
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 in Knocked Up's final cut but as an extra on the DVD, the scene goes on for over six minutes.
21. Good Will Hunting - Sean Maguire Tells a Story
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 Good Will Hunting script.
20. The Dark Knight - Nurse Joker Walks Away
Returning to The Dark Knight, 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.
19. Aliens - Game Over Man
Chaos and confusion are everywhere after the first attack by the xenomorphs in Aliens 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.
18. Tootsie - Bill Murray: Natural Entertainer
During this Tootsie 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.
17. The Usual Suspects - The Line-Up
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 Usual Suspects' DVD, the laughing during Del Toro's delivery was due to his constant farting while filming - boys will be boys.