Un professeur d'américain (1978)

An idle man tries to kill the time by following strangers down the street, so as to know everything about their lives. An ordinary-looking American teacher catches his eye and he starts following him, making no attempt to conceal his maneuver. Eventually, the professor becomes exasperated and decides to talk to him. Knowing he can't get rid of the intruder, he hires him as his driver. The follower is now part of the professor's retinue. It's now up to the professor to appear strange to the man.