PARK CITY - Some small movies are bigger than others, and few contemporary filmmakers' careers are better suited to that sliding scale than Joe Swanberg, the self-sufficient indie all-rounder who has quietly reeled off 16 feature films since 2005. Until recently, they've been uniformly scrappy in scope and construction, with some more considered than others: the personal, plainly self-reflexive relationship studies (2008's Greta Gerwig-starring "Nights and Weekends" was a standout) rather than the quick-sketch genre exercises.

Happily, it's in the former rather than the latter mode that Swanberg has been edging slowly toward the mainstream. Last year's "Drinking Buddies," a smooth sort-of-romantic-comedy built around a sort-of-star in Olivia Wilde, converted a lot of skeptics to Swanberg's cause. More still will be lured by the disarming "Happy Christmas," which peels back a little of its predecessor's polish while extending its command of centered comic character study: gentle and generous and gifted with 35mm texture, it might be Swanberg's warmest, most easily enjoyable film to date.

To explain what happens in "Happy Christmas" isn't really to explain the film at all -- we're still in Swanberg's favored realm of epiosodic non-narrative here, though the episodes themselves are fuller than usual, more alive with human business. Shot in fleet, candid fashion in Swanberg's own Chicago household, its short-story premise is simple enough to sustain the jostling improv energies of five very different actors.

Swanberg's preference for freeform riffing can seem exercise-y in other contexts; here, the underlying sense of smart actors gamely feeling each other out reflects certain polite uncertainties within the character network. An affectionate but tellingly nervous chemistry hovers even in tender scenes between Kelly (Melanie Lynskey) and Jeff (Swanberg), a semi-hip Chicago couple whose happily married status doesn't preclude certain blind spots in their understanding of each other, particularly since the birth of their infant son Jude (played, in hilariously responsive fashion, by Swanberg's own mini-me of the same name). Filmmaker Jeff is too besotted with the baby to notice his wife's post-partum restlessness; Kelly, an inactive novelist, is too guilt-stricken by her sense of less-than-complete maternal bliss to let him know otherwise.

The unlikely catalyst to this comfortable but static relationship arrives in the slouchy form of Jeff's kid sister Jenny (Anna Kendrick), who throws herself on their mercy (or at least their basement couch) after an especially frazzling breakup. An overgrown adolescent who uses her emotional state a little too readily as an alibi for reckless binge-drinking and job-stalling -- egged on by her flaky college pal Carson (an amusing and tidily cast Lena Dunham) -- Jenny is at once an irritation and a necessary challenger to the couple's routine.

Rather like "Junebug," another lovely film built on sister-in-law relations, no one in this triangle is a purely oppressive or redemptive presence; Swanberg is a humane enough dramatist to give no character the moral upper hand, even if some make better decisions than others. Shy, mostly strait-laced Kelly learns from Jenny that a little selfishness never did anyone any harm -- desperate for some alone time to pursue her next novel, she's astonished to find she need only ask for it. That's not to say, however, that Jenny's own selfish impulses make her any less of a pill, either to her family or her affable, pot-scented new boyfriend (Mark Webber). "Happy Christmas" is a film about growing up, to some extent, but it's not so dull as to suggest everyone can -- or should -- grow to exactly the same point.

Swanberg claims to have drawn this slender story from his own marriage and sibling relationships, and his personal investment in the material could seem noodly or navel-gazing if it didn't touch on broader truths about the way adults alternately grow out of and into their families. ("Beasts of the Southern Wild" DP Ben Richardson's nosy-cosy camera probes the cluttered corners of Casa Swanberg -- with its melamine kitsch sensibility and built-in tiki bar -- as if shooting an artful and particularly niche episode of "MTV Cribs.")

It's Swanberg's co-stars, however, who make "Happy Christmas" moving and not merely intimate. Lynskey, sporting her native Kiwi accent and the same diffident intelligence that made her so endearing in Todd Louiso's "Hello I Must Be Going," has a knack for zeroing in on heartbreaking details of gesture and posture that betray Kelly's shifting sense of self-worth. (She also gives a wincingly accurate impression of writerly self-distraction.) Equally neurotic, but at a more jittery pace, Kendrick is her ideal comic foil; watching the two women meet each other halfway is one of several subtle pleasures in "Happy Christmas," a film in which lives are richly altered even as nothing ostensibly happens.