Charlie Brown is a model neurotic. He is prone to depression and anxiety and paralyzing fits of over-analysis. Constantly worrying if he is liked or respected, he has a perpetual, usually dormant crush on the little redheaded girl, taking small joys in her foibles (like biting her pencil) that may make her more attainable. He is noted for his inability to fly a kite.
Snoopy = Extraversion
Snoopy is a typical extravert. Flamboyant, daring, and outgoing to a fault, he tries to join in every activity and conversation. He (perhaps fictitiously) flies gallant missions against the Red Baron and then brags about his exploits. For reasons potentially stemming from his long-ago abandonment of his mother, he aggressively pursues friendship and food (not in that order). Snoopy is Joe Cool, the life of the party.
Lucy = (Dis)agreeableness
Defined by a single word (crabby), Lucy revels in her disagreeableness. Typical portrayals of Lucy feature her bossing around her friends, dominating her little brother, mocking Charlie Brown's self consciousness, and generally being a pain in the ass. Her attempts at psychiatry generally involve misguided advice delivered loudly and angrily. One recurring interaction is Lucy pretending to hold a football out for Charlie Brown to kick, and then pulling it out at the last minute. Brown goes thump and Lucy preens.
Linus = Openness to experience
Linus is clearly the brightest of all of the Peanuts gang. Witty and knowledgeable, he is prone to passionate monologues. He has invented his own creation, the Great Pumpkin, and faithfully waits in the pumpkin patches for him every Halloween. Linus has his own idiosyncrasy, an ever-present blue security blanket - but he does not seem particularly sensitive about it; it's who he is. Too young to active try new things, he must instead use his intellect to mull over new and interesting ideas.
Schroeder = Conscientiousness
Charlie Brown, Linus, Snoopy, and even Lucy are fairly well-developed characters. Schroeder is equally (perhaps more) lovable, but most casual readers know him for one thing: his piano playing. Yes, Lucy has a crush on him, but that's about her - he will have none of it. He is always practicing. Disciplined and focused on his passion for classical music, one can imagine him setting his alarm clock for seven a.m. on weekends to try Autumn Sonata one more time. His one other preferred activity is playing catcher for the baseball team - again, the sturdy, reliable director of the action on the field. Schroeder would offer to help you move and show up ten minutes early.