Need help finding a movie

A friend asked me about an old movie we once saw, but neither of us can remember what it was. As soon as he described the one scene, I could see it in my mind, but can't see the actors faces or remember the movie itself... So the scene goes like this: There was a movie shooting, and one of the actors started to stab the other one killing him, and the cast thought that this was just a good act, same for the production team, but he was just killing the other guy. If anyone knows what this movie is, can you please tell me? Many Thanks