Zoolander
Try to remember this genius scene in Zoolander. In the original script, David Duchovny explains to Stiller the reason why he thinks male models would be exceptional at carrying out assassinations, but Stiller asking, “Why male models?” was off the cuff. Anyway, Duchovny went along with it, and the outcome was way better than anybody could hope for!
The Shining
The bit in The Shining with “Here’s Johnny!” must have been everybody’s first real taste of horror! This bit might be really clever but you won’t find that line anywhere in the script. To pay tribute to The Tonight Show with Johnny Carson, Jack Nicholson ad-libbed that line.