Nearly all of the angst in “Thumbsucker” is internal. It is not a film about how people relate to each other, but how they relate to themselves. Yet despite the introspection, it’s an accessible movie, a colorful and warm coming-of-age story with rich characters. If it sometimes overdoes its indie-film quirkiness, it ultimately makes up for it with real depth and laugh-out-loud comedy.

The title character is Justin Cobb (Lou Pucci), 17 years old and uncomfortable with himself, as all teenagers are, but especially those who are different. Justin’s difference is that he sucks his thumb when he is stressed. His strangely unsupportive father Mike (Vincent D’Onofrio) is annoyed by it. His mother, Audrey (Tilda Swinton), is more sympathetic but is preoccupied with her obsession with movie star Matt Schramm (Benjamin Bratt), who has just checked into the rehab facility where Audrey works. (Justin calls his parents by their first names, by the way, an example of the film’s over-reaching quirkiness.)

The school psychologist diagnoses Justin with ADHD, and medication seems to help. He begins to excel in the debate club and reads “Moby Dick,” a feat which few people even of normal attention spans are able to accomplish.

As Justin comes to terms with himself, we realize his thumbsucking is a metaphor for whatever makes us different, and that different is not necessarily bad. The medication reduces the thumbsucking, but it also reduces some of who Justin is.

He encounters a number of “different” people in his travels, including Vince Vaughn as the too-cool, eager-to-please debate coach (he buys booze for the kids when they’re away at a tournament), and Keanu Reeves as a New Age dentist. Vaughn and Reeves are both very funny, with Keanu walking the line between being his usual bad-actor self and making fun of his usual bad-actor self.

Eventually Justin is confronted by his little brother Joel (Chase Offerle), who says, “You ever stop to think you’re so busy being weird that I have to step up and be normal?” Indeed, so much of Justin’s woe could be solved by accepting his thumbsucking tendencies, rather than fighting them. Would it really be so awful if he sucked his thumb for the rest of his life?

Lou Pucci gives one of the most likable, charismatic teen performances in recent memory. His Justin is earnest, smart and, most importantly, believable. Most of the film’s comedy (written and directed by Mike Mills in his feature debut) stems from situations that are exaggerated, perhaps, but not stretched beyond credibility. Pucci plays it with heart and natural ability — not unlike Aaron Stanford in “Tadpole,” which was a similarly oddball coming-of-age story that played at Sundance a few years ago. I suspect Pucci will be one to watch in the next few years, and “Thumbsucker” is an auspicious start.

The film makes two points, though. The more obvious one is that being different is not necessarily bad. But the other one is this: Not having all the answers isn’t necessarily bad, either. Justin, his parents, his New Age dentist — each of them is making desperate attempts to discover what life is all about, to find “the answers.” What the wise ones eventually learn is that maybe they don’t know anything, and maybe that’s OK. You can be uncertain and still be happy.

B+ (1 hr., 34 min.; R, a few F-words, a little sexuality, a brief violent image.)