SpikeTv listed Classic Movie Moments You Didn't Know Were Improv. I've never seen Full Metal Jacket, Taxi Driver, or Clockwork Orange so I skipped theirs. And the 'Here's Johnny!' (The Shining) and 'Play it Again, Sam' (Casablanca) are already well known.
So below are just my favorites from their list.
First, from 'Raiders of the Lost Ark':
''Egypt was absolute hell to shoot, with cast and crew frequently becoming ill and injured. At one point Harrison Ford tore a ligament and John Rhys-Davis actually crapped his pants during filming. Much of it was pushed through and indeed the entire scene with Marion (Karen Allen) and Belloq getting drunk in a tent was also completely improvised.
The sequence with the swordsman was originally meant to involve Indy running around with the swordsman in pursuit, but Ford had come down with dysentery and wasn't in the mood to do much running when his bowels were doing enough of that already. So he suggested they just shoot the bastard and be done, resulting in one of the most entertaining scenes in the entire film.''
Indy Whips Up An Ad-lib ;-)
Raiders: Marion and evil Belloq get drunk
Teeth-sucking Cannibal Hannibal
''While this line was in fact in the script, his hissing after was not, and surprised everyone on set particularly Jodie Foster. That disturbed look on her face there? That wasn't acting - she was genuinely creeped out.''
Congrats, Anthony Hopkins...
That scene is still creepy to me too.