Tammy Melissa McCarthy, Susan Sarandon, Mark Duplass. Directed by Ben Falcone. Rated R. Now playing.

In a way, Melissa McCarthy has become the victim of her own success. Thanks to her popular performances in movies like Bridesmaids, The Heat and Identity Thief, audiences expect to see the same vulgar, loudmouthed character from her in every new movie, and the marketing for Tammy has promised exactly that. But while there are a few over-the-top moments in Tammy, it’s much more low-key than McCarthy’s past hits, with a shapeless, character-driven story about the reluctant bond between the title character (McCarthy) and her irascible grandmother Pearl (Susan Sarandon).

After losing her job and her marriage in the same day, Tammy decides to hit the road, and the only way she can get a ride is to agree to take Pearl along on her ill-defined adventure. At times, Tammy resembles a broader, gender-reversed version of Nebraska, and McCarthy has the acting skills to pull off an affecting indie dramedy. But Tammy is still a mainstream comedy, so it’s beholden to its wacky set pieces, and the heartfelt moments come off mostly as afterthoughts. McCarthy and her husband Ben Falcone (who co-wrote and directed) spent years working to get Tammy made, but it’s hard to see what qualifies this modest, meandering comedy as a passion project.