Steven Greydanus on the red flag scene in Modern Times:
Witness the subtlety and comic timing of the scene in which the Tramp is arrested for communist agitation. Just released from a psychiatric ward, the Tramp happens to see a flatbed truck with a long load, trailing an obligatory red warning flag at the end. When the flag falls off the back of the load, the Tramp helpfully scoops it up, waving it to try to get the driver’s attention — not realizing that a throng of unemployed workers from his old job has come up behind him, demonstrating in the streets. To the police, of course, the Tramp waving his red flag at the head of the crowd looks like the leader of these agitators; and he is quickly bundled off to jail.
Note how gracefully Chaplin weaves together the demands of (a) his medium (although the film is black and white, we know the flag is red because it comes from the back of the long load on the flatbed), (b) his comedy-of-errors genre (an innocent attempt to help is mistaken for political agitation — just the kind of thing that would happen to the Tramp), (c) the continuity of his story (the demonstration doesn’t simply appear from nowhere for the sake of the gag, but flows naturally and logically from past events), and (d) the social concerns underlying the film (the workers’ plight mirrors real problems — as does that of the Tramp, neither the first nor the last to be wrongly persecuted for the belief that he is a Communist).