Remember that scene in CASABLANCA when the piano player in Rick’s Cafe turns to look at Rick before playing the accompaniment to the Marseillaise, the French national anthem? Rick gives an almost imperceptible nod. Rick’s nod was a small movement, but it set the course for the rest of the movie. Up until then we weren’t quite sure which side, that of the French Resistance or the occupying Nazis, Rick would choose. And he had to choose. In that time and place there was no truly neutral position available.
I’ve read that the pivotal scene described above was an afterthought. The scene was actually a call back to Humphrey Bogart who thought he’d finished shooting the film. No other actors were needed. Bogart stood alone on the set, in the shadows at the edge of the room and nodded. The scene was cut into the film at the proper place and history was made.