The following excerpt from TCM's article on Louis Malle's LACOMBE, LUCIEN provides some interesting information on the lead actor in the movie, Pierre Blaise:
Malle would be the first to admit that much of the picture's ultimate impact was due to its star, a rather nonchalant amateur named Pierre Blaise. Malle knew that the character had to be played by a man from the area where the story takes place, and he had to have an accurate accent. So Malle started fishing for a performer during a series of casting calls that didn't go well. That is, until Blaise walked in.
Malle tells [interviewer Philip] French that, as he was walking out of the casting building, he ran into Blaise. The boy, who had a tough peasant accent, explained that he was there for an audition, but had arrived late. "Immediately, I saw something about him that was unique," Malle said. I said, 'Let's go to a caf and talk.' I discovered he'd been more or less forced by his mother to come; he had absolutely no interest in playing the part." The more Malle talked to Blaise, the more he grew convinced that he had found his Lucien.
But Blaise wasn't entirely taken with the filmmaking process when shooting began. He told Malle after the first few days of filming that he was going home. Malle and an assistant had to talk him into sticking around. Malle later determined that Blaise, who was only 17 at the time, didn't like how everyone on the set ordered him around all day long. Malle gathered his main technical collaborators together, and said, "...'Starting Monday, you're going to treat him as Alain Delon. Don't think of him as Pierre Blaise, this little peasant of seventeen. Think of him as Belmondo. You have to be really cautious. He's got the whole film on his shoulders; he's so much more important than any one of us.' And from then on, things went better. He started enjoying being the main man on the set."