1. The Joker's slow clap was Heath Ledger's genius in The Dark Knight.It was a response to Jim Gordon's victory speech, making his character become an unpredictable villain.

2. "You talking to me?" from Taxi Driver. Robert De Niro improvised the entire scene where he's talking to himself in the mirror. One of the most iconic scenes in movies ever.

3. Jim Carrey's road trip antics were unscripted in Dumb and Dumber. Jeff Daniel's hysterical laughter too.

4. Don Corleone's cat-stroking in The Godfather was one of the greatest improv scene ever. A stray cat was found wandering on set, and Brando just came up with the idea of gently stroking it.

5. "Here's Johnny!" in The Shining done by Jack Nicholson tormenting his wife and child with an ax.

6. Albie's sneeze during the coke scene in "Annie Hall" wasn't planned.

7. The fake orgasm in When Harry Met Sally. Meg Ryan came out with the idea to do it and it coined the line, "I'll have what she's having."

8. The necklace box snap in Pretty Woman started as a practical joke, but was one of the best scenes in the movie.

9. The Hiss in Silence of the Lambs. Think Hannibal Lector was scary? He became even more scary when he did the terrifying hiss sound.

10. Han Solo's response to "I Love You" in The Empire Strikes Back. His classic line, "I know," responding to Leia's declaration of love. The scripted line was supposed to be, "I love you too."

11. Robin William's farting wife in Good Will Hunting wasn't planned.

12. "I don't care" in The Fugitive. The showdown between Kimble (Harrison Ford), accused of murdering his wife, and Gerard (Tommy Lee Jones) was one of the best improvs ever. Kimble pleased, "I didn't kill my wife." To which Gerard replied, "I don't care."

13. Singing in the rain in A Clockwork Orange didn't happen because it was planned. It was because Malcolm McDowell was frustrated having to shoot the scene so many different times that he belted out "Singing in The Rain" during a harsh act of violence.

14. John Malkovich hit by a can in Being John Malkovich was not planned. It was thrown by a drunk extra, and Spike Jonze left it because it showed Malkovich's vulnerability and frustration.

15. "Why male model?" in Zoolander. The first time he was asked that question, it was completely scripted, but Stiller forgot his next line, causing him to repeat his last line, "Why male models?" David Duchovny, playing Prewitt, ad-libbed "Are you serious? I just told you that a moment ago." Resulting in a hilarious scene enforcing the utter stupidity of Stiller's character.