All the young actors who unsuccessfully auditioned for the role of Antoine were used in the classroom scenes.
So pleased with Jean-Pierre Léaud and his screen test (an informal conversation with the film's director being off-camera), François Truffaut doctored it into the finished film by using fade-outs and substituting his voice with off-camera female psychiatrist's voice.
The English title of the movie "400 Blows" is an overly literal translation of the original title. The original title stems from the French expression "Faire les quatre cents coups", a figure of speech meaning "to live a wild life", as the main character does. A better translation of the expression would be "to do the 400 dirty tricks".
Jean-Pierre Léaud's answers to the questions given to him by the psychologist at the camp near the end of the film were not scripted. François Truffaut told Leaud in advance about the scene for what to expect to a certain extent, and did provide some minor coaching when Leaud answered the question in between takes as to what was working and what was not, but at large, Leaud's answers are unscripted and ad libbed, per Truffaut's wishes, who wanted the scene to feel spontaneous and believable.
All spoken lines in the film are dubbed over again by the actors themselves, save for a few minor and trivial parts. For instance, during the last scene, the sound of Antoine's footsteps was added during editing - the truck that the camera rested upon produced too much noise. Shooting on the streets of Paris, as many films of the French New Wave did, was often hectic and re-dubbing everything allowed François Truffaut to not have to worry about lugging bulky and expensive sound equipment around, and more importantly he would not have to worry about a street scene having too much background noise. This made shooting faster and easier.
François Truffaut: is seen riding next to Antoine in the centrifuge ride at the fair, and can then be seen smoking a cigarette just outside the ride.