I think of Joe Swanberg’s latest lo-fi indie comedy as less a successful movie than a successful situation that happens to take place in a movie. I like it, maybe more than his prolific progression from mumblecore to auteur-dom (Hannah Takes the Stairs, Drinking Buddies, etc.), but I increasingly doubt he’ll mature into a filmmaker who can tell complete stories. Or maybe that’s an obsolete notion in the age of Twitter, like talking about the Great American Novel. You get the sense that no one here is aiming that high. Happy Christmasis a genial but very low-stakes enterprise, where you can guess the last scene by the time the first scene’s done (and not much separates the two).

Anna Kendrick, cast against her brainy, high-strung type (Rocket Science, Up in the Air) plays Jenny, a directionless and self-destructive 27-year-old who crashes at the Chicago homestead of her older brother. Jeff (Swanberg) and his wife Kelly (Melanie Lynskey) have a toddler (played by Swanberg’s own son, a wildly charismatic 2-year-old named Jude). The fun, or at least spontaneity, has gone out of this exhausted household. Jenny is the needy, disruptive force, a pothead and binge drinker, who’ll shake things up. Funny and dry, Lena Dunham is her comparatively restrained wingwoman, Carson.

All these characters, genius Jude excepted, are operating well below potential, with Jenny the queen of the slackers. She tries to enlist Kelly, a frustrated novelist, into co-authoring a trashy romance novel, leading them into a long meandering talk about sexual euphemisms with Carson. Swanberg has a generous approach to such ordinary life fodder; the millennial-generation anxiety he gets right, but he never sharpens the writing to conclusive punch lines or plot points. (The cast actually improvised much of the script.) During the summer doldrums, that’s a relief from Hercules or The Expendables 3, where everything moves mechanically forward to the next scene (or explosion). Happy Christmas can’t be bothered with that. It’s too relaxed for its own good, too unambitious, yet it still leaves you smiling in the end. Opens Fri., Aug. 8 at Varsity. Rated R. 82 minutes.

bmiller@seattleweekly.com