Marlo, Charlize Theron’s lead character in Tully, fends off small talk with barrages of acid-dipped put-downs, and dismisses anything sentimental as corny. So you wonder what she would think of her own film, which conceals a tender heart within an outer skin of sandpaper.

That’s not a knock; Tully makes hipster sincerity look good. Its approach is the modus operandi of screenwriter Diablo Cody and director Jason Reitman. Their 2007 film Juno also used pregnancy as its jumping-off point, before coasting along on its cutesy one-liners and very conventional resolution. Thankfully, Tully is thornier and wearier, with an authentic sense of both dejection and hope.

You can see why Marlo’s defense mechanism runs hot. Ignored by a husband who retreats into video games, pregnant with her third child, and struggling mightily to understand a son with special needs, she must also bear the questioning gazes around her. Her second pregnancy brought on postpartum depression. What’s going to happen this time? Theron, who previously worked with Cody and Reitman on Young Adult, embodies every ounce of Marlo’s weariness (and her quippy attitude). She always looks as though she’s plodding around in an old unbelted bathrobe, even when she’s not. This is one of those rare times that an actor’s weight gain is truly justified, even more so than Theron’s Oscar-winning turn as the beefy killer in Monster. This film’s depiction of motherhood is so intensely physical, it makes sense for Marlo to look as though she’s moving around in an unfamiliar wrapper. “What’s wrong with your body?” Marlo’s daughter asks her. She doesn’t answer.

Then things change. Marlo’s financially successful brother (Mark Duplass) and irritatingly chipper wife (Elaine Tan) offer a baby gift: picking up the tab for a “night nanny,” someone who stays overnight for a few weeks and tends to baby so the parents (reliable Ron Livingston plays Marlo’s zoned-out husband) can get a reasonable amount of sleep. Marlo’s nanny is Tully (the very-nearly-weightless Mackenzie Davis, from Blade Runner 2049), a remarkable young woman who waltzes in like some fantastical blend of Mary Poppins and Oprah Winfrey. She doesn’t just look after the new baby, she provides wise counsel and bakes a mean batch of cupcakes.

Her arrival sparks something dormant in Marlo, and for a while the movie is touched by a Frank Capra-like magic, as though an angel had dropped down to prove that it’s a wonderful life after all. Things don’t work out the way we expect, as the story moves toward an ending that might warrant Marlo’s disapproval, but seems entirely earned.

Cody could write caustic jokes all day if she wanted to; Marlo has a great moment when she cruises past a beloved former bakery and wistfully sighs, “People ate flour back then.” But this movie doesn’t settle for the one-liners. This time Cody is more invested in building a credible emotional life for her protagonist, and Theron is right there to handle the heavy lifting.

Tully

Now playing | Rated R

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