A prime example, writes Roger Clarke in The Independent, came with The Raiders Of The Lost Ark, written by George Lucas, Philip Kaufman and Lawrence Kasdan.
In one of the most famous scenes in the movie, an assassin dressed in black confronts Indiana in a souk in Cairo. Marion, played by Karen Allen, is about to be abducted after she unwisely hides in a convenient rattan basket. Indy hasn't much time. But the looming Arab swordsman has plenty of time. He loops his gleaming scimitar in skilful arcs of light. He is going to take great delight in showing his skill at killing this infidel American. Indy, on the other hand, doesn't even bother to reach for his bullwhip, his usual weapon of choice. Without the merest hint of fair play, he shoots the swordsman down with his pistol.
As it happens, the script had allowed for an extended period of choreographed fighting. But Harrison Ford had other things on his mind. He needed to use the lavatory. He had diarrhoea. This comically improvised scene proved such a hit with the director, he junked the written scene and left it in.