Well, for most formal portrait photographs just about anywhere in the world during this time, it seemed like you were expected to put on an expression like you had just passed a porcupine...
That’s what gives the film clips their special informal, spontaneous quality.
Portrait photographs often used exposure times measured in seconds. Any movement would blur the shot. Maybe some people could hold a smile sufficiently motionless to take a good picture, but less cheerful expressions were easier to keep still.