“Very often when an actor forgets his lines, it is because a voice inside his brain has whispered to him, ‘Wouldn’t it be dreadful if you forgot the lines?’ And that voice has generally entered the brain at the moment the actor loses contact, however momentarily, with the character in the situation, and, looking back to examine this or that line, turns, like Lot’s wife, into salt.”
Simon Callow, Charles Laughton: A Difficult Actor