Jean-Pierre Léaud did not receive an Oscar nomination for portraying Antoine Doinel in The 400 Blows.
Jean-Pierre Léaud plays that young boy who leads the film and it is essential that Léaud passes the child actor test for a realistic film which is to be believable as a kid actually. Well Léaud certainly meets this requirement for Antoine. Although he leads the film though this is not a precocious or endearing young child that we are going to follow. In fact Léaud gives a fairly uncharismatic performance, but don't take that as a criticism. Léaud does not make Antoine an endearing little boy who we're going to enjoy having adventures with. Although really some of the stuff he does is often construed as such in other more lighthearted films that is not the case here. Léaud instead portrays him as the rather unpleasant child that he is. Léaud captures that almost perpetual pout of such a child who always seems slightly at unease even though there is nothing specifically causing him pain. After all he has a family who technically provide for him, he goes to a nice enough school, he even has friends, but nevertheless Léaud portrays the boy as never really being happy.
Léaud's work exudes that sort of indifference of Antoine's behavior in his life. When he behaves poorly in school there is nothing particularly funny about anything he does. He's not doing it for enjoyment he's just doing it. The same goes for the lies that Antoine constantly tells. Léaud never depicts any shrewdness in this, there is not a hint of mischief in it either. Instead he portrays it as a bit of blank action of sorts that again is something that Antoine just does. Even when he lies to his teacher by saying his mother has died in order to explain his absence there is something quite lifeless about the way Léaud delivers in these scenes. That's even the case when he steals, there's Again I am not criticizing his performance at all when I say this, this works instead to accurately show the behavior as really meaningless behavior. Well meaningless in what he's trying to get out of it in the short term, but not meaningless altogether. Léaud does well to allude to the need in Antoine for attention driving this though in a subconscious fashion.
This seems to have developed from Antoine's relationship with his mother, who was unwed when he was born, and did not raise him for many years of his childhood. The problems are compounded through her most recent behavior to him which is quite random as she will become quite cruel one minute than excessively encouraging the next in order to comfort him. Léaud is good in these scenes between Antoine and his mother as he expresses the awkwardness of their interactions. They never quite seem to get along, and even in their moments of warmth there is still something problematic about it. Léaud never depicts a full contentment with Antoine towards his mother as though he's unable to fully understand her own problematic behavior as well as can't quite reconcile her past abandonment of him. They is always that barrier that also extends to his step-father, unfortunately because he is his step father, because Léaud suggests a little more comfort with him as there are not those lingering feelings of betrayal when the two speak with one another.
Although his behavior often is pointless and in general there is a cold demeanor about Antoine, Léaud never makes him emotionless. Importantly because of suggesting where this coldness comes from, but also he shows a bit of difference in himself when he is with his peers. In these moments there is more of an investment he shows, and when he is directly embarrassed in front of them Léaud shows a greater vulnerability in the boy. The friendship he has with another boy clearly matters to him, and there is a very affecting scene for Léaud late in the film when he is pained to see that his friend is not allowed to see him. There is another more open sequence, that might be where the majority of his lines in the film comes from, where he goes to see a psychologist who asks him various things about his life. Léaud uses the scene well to present more overtly though troubled feelings that compel his behavior, but also that even behind his stare that he's just a boy through his shyness when asked if he's ever slept with woman. This is a good performance by Léaud as he simply accurately depicts this sort of child, not as a psychopath, or an elvish scamp, but just as a deeply troubled boy.