Nick Cassavetes’ ‘The Other Woman’ has been sold as a female revenge-comedy, but for much of the mostly muted first hour, the film is a story about what happens when career-minded Carly (Cameron Diaz) discovers she’s the other woman in her man’s life, and befriends her beau’s wife. The movie quickly -- and smartly -- goes from being Carly’s story to being the story of Kate, played beautifully and with heartbreaking hilarity by Leslie Mann, best known to audiences as Judd Apatow’s wife and an actress who frequently appears as a wife in Apatow’s films.

While ‘The Other Woman’ devolves in the second act, sidetracked with gross-out humor and Kate Upton’s unfortunately one-dimensional ditzy second mistress, it’s Leslie Mann who continually shoulders the film. Diaz vacillates, often as unsure of her character as the script seemingly is, waffling between confident lawyer (every time someone enters her office, the camera makes sure to zoom in on the law firm’s sign so we are reminded that she is a successful lawyer) and scorned, vulnerable woman learning to soften her heart … or something. But Mann is all confidence in the less than confident character of Kate King, wife of pathological cheater Mark (‘Game of Thrones’ star Nikolaj Coster-Waldau; and if you’ve ever wanted to see Jaime Lannister with explosive diarrhea, here’s your chance).

We’re most familiar with Mann as the no-nonsense wife and modern mother, put-upon by her husbands in Judd Apatow’s films, basically playing what we assume is a variation or heightened version of her real-life self. She also played a wife and mother in the horrendous body-swap comedy ‘The Change-Up,’ in which she was saddled with a troubling and baffling CGI set of breasts, and played what once again amounted to something akin to her Apatow characters, presumably the reason for which she was cast.

In ‘The Other Woman,’ Mann shows off her versatility and range, giving us something much different than we’ve seen from her previously, bringing real humanity and empathy to the character of Kate -- a woman who compromised her own dreams for those of her husband and has been relegated to the role of housewife, supplying her husband with ideas for his job, but never fully reaping the rewards. It’s a cautionary tale for all women, and a nice inverse to Diaz’s strictly professional Carly. Mann relies on facial expressions and contorting her body in strange and hilarious ways, milking every drop of humor out of each scene, but it’s the moments in which Kate is heartbroken, confused, lost and spiraling out of control that Mann is at her best, and she’s able to pull off the miraculous feat of making these moments both sad and hilarious, finding the humor in the heartbreak. We feel as though we’re laughing with her because we’ve all been Kate King in one way or another, and this becomes even more painfully accurate as the film wears on and Kate contemplates staying with her husband, who truly dotes on her and seems to really love her, but shows no signs of changing his cheating ways -- is he capable of change? Can their relationship be healed? Can she forgive and forget? Should she still sleep with someone who has so fully and flagrantly broken her heart?

The complexity in Kate’s ongoing conflict (and the immediate understanding by all women that Mark is to blame, not them) is as close as ‘The Other Woman’ gets to being real -- and a lot of that is thanks to Mann’s ability to be sympathetic and relatable, unlike Diaz and Upton, who will always translate on screen as just a little too perfect and put-together. Where Mann often feels as though she’s playing a heightened version of herself in other films, Diaz and Upton have that aspirational star quality where, regardless of the names they’re given in this film, they’re still Cameron Diaz and Kate Upton. Even Googling the film, the most frequent image results are photos of Diaz, Upton, and supporting star Nicki Minaj, who's basically playing the minor part Kim Kardashian played in Tyler Perry's 'Temptation: Confessions of a Marriage Counselor' -- and just as poorly and robotically acted. It's a shame because this is, without a doubt, Mann's movie. Without Mann, ‘The Other Woman’ would hardly be bearable -- just imagine how great she could be in a non-Apatow female comedy with a better, more consistent script.