Paul Rudd is a national treasure. His smaller roles were a highlight of movies like “The 40 Year Old Virgin,” “Knocked Up,” and “Anchorman.” He was charming in “Clueless,” heartbreaking in “The Object of My Affection” and “The Shape of Things,” and downright hilarious in a brief cameo as John Lennon in “Walk Hard.” This movie seems to be about the big, loud, dumb, humiliating moments — an unexpected same-sex kiss in public, an embarrassing photo made public, an intimate moment made public — but Rudd is at his best in the small, perfectly timed moments when his character is trying hard to be a “regular guy,” even though in his heart he suspects that whatever it takes, it is beyond him.

Whether it’s a romance or a bromance, movies about couples almost always position them as superego and id. On one side is a responsible, mature, thoughtful person who follows the rules. On the other is someone who is impulsive, outspoken, and a lot of fun but not quite a grown-up. Generally, both discover that they are missing something and it all ends happily ever after.

The romantic couple in this movie are both pretty much in the responsible and rule-following category. In its first moments, Peter (Paul Rudd) proposes to Zooey (Rashida Jones). She accepts. And then she immediately has to share the moment with her nearest and dearest — her friends. On the way home from the proposal she puts them on speakerphone and they do everything but launch into a conference call version of “Going to the Chapel” before inadvertently revealing to Peter that Zooey has kept them privy to the most intimate (and I mean intimate — and privy) aspects of their 8-month relationship.

Peter now has everything, a wonderful fiancee and some great career prospects in real estate (if he can just sell Lou Ferrigno’s house to get the investment capital). But when he has no one to call to share the news of the engagement and he overhears Zooey worrying that he will be too dependent on her, Peter decides he has to find a male best friend. He goes on some “man-dates” but no one is right. And then he meets Sydney (Jason Seigel of “How I Met Your Mother” and “Forgetting Sarah Marshall”), whose unabashed honesty, easy acceptance of Peter’s floundering attempts at guy-talk, and endless time for hanging out make Peter feel at home. Sydney lives in a “man-cave” but he is in essence a boy, a cross between Peter Pan and Lampwick, the kid in “Pinnochio” who turned into a donkey on Pleasure Island.

The film shrewdly salutes and makes fun of the way that the progress toward friendship parallels a romantic relationship, the attraction, the tension in the initial invitations, the thrill of finding what you have in common (they both love Rush!, the sweetness of feeling completely at home with someone. Rudd is terrific as we see him trying hard to interact with Sydney, for whom male friendships come naturally. The expression on his face as he tries to match Sydney’s comfortable conversational rhythms is a gem of comic mingling of anxiety and pleasure. There is some nice understated support from Andy Samberg as Peter’s brother and J.K. Simmons (“Juno”) as his father, but the shrillness of the stereotyped hostile married couple played by Jon Favreau and Jaime Pressly is annoying. Thankfully, the rest of the movie avoids the usual terror of women usually found in man-boy comedies, but it would be nice to let the girls get a laugh once in a while, especially when they are as talented as Pressly and Jones. The script is predictable and the movie falls apart whenever Rudd is off-screen, but as soon as he returns he makes it watchable and even endearing.