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 a gross misinterpretation of the original title. The Finnish and Swedish translations of the title, roughly translatable to "400 practical jokes" are closer to the original meaning, albeit not perfect. The Swedish title: "De 400 slagen" means "The 400 blows" and make no sense. The original title stems from the French expression "Faire les quatre cents coups", meaning "to live a wild life", as the main character does. Literal translation of the expression would be "to do the 400 dirty tricks".
Jean-Pierre Leaud's answers to the questions given to him by the psychologist at the camp near the end of the film were not scripted. Francois 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.
When Antoine and Rene are suspended from school, they go gallivanting around the town. At one point they pass a wall of posters and flyers, and they pull off a picture of a woman. The woman is Harriet Andersson in a shot from Ingmar Bergman's Summer with Monika (1953), also about two young lovers who ran away from home to "live their own life."
When Antoine's father finally nodded to the cinema business, realizing it is Rivette's "Paris nous appartient", he said: "Si c'est un complot..." And a 'complot' (conspiracy), really, is the central issue of the Rivette's film.
The Spanish (Spain) dubbed version of this film has about 12 minutes of footage missing. When Antoine's mom returns home and argues with her husband while Antoine pretends to sleep, the scene in which the family heads home after going to the movie theater, when Antoine and René smoke and drink in Rene's room and when they throw things from the ceiling with blow pipes, are among the scenes that are missing. The scene in which the father talks about the new secretary sleeping with the boss is dubbed to the father speaking about the boss liking the new secretary and her being a very good worker and being promoted because of that. The interview with the psychologist was dubbed with the psychologist asking Antoine if he has had a girlfriend, and he talks about dating some girls but not liking any of them and finding a girl he liked but who chose an older guy instead of him, when in the original he is asked if he's slept with a woman and he goes on to talk about when he tried to get one to sleep with.