The best reason to see Karyn Kusama’s Girlfight is for the surly, indrawn expressiveness of its star, Michelle Rodriguez. She plays Diana Guzman, who lives in the projects in Brooklyn and can’t find an outlet for her aggressions until she starts boxing lessons at a local gym. There have been other recent movies about women fighters, including Shadow Boxers and On the Ropes, but those were documentaries. Through Rodriguez’s performance, Girlfight brings into the fictional realm some of that caught-in-the-moment documentary realism; Rodriguez had never boxed before nor acted except as an extra, and she seems to be thinking through every move she makes. Yet her acting is also sensual and instinctive. Her performance is an amateur triumph, but the same can’t really be said for the film, which doesn’t know what to do with Diana’s unruliness except to channel it into a feminist fable of empowerment in which the girl is finally forced to square off against her boyfriend (Santiago Douglas) in a metaphorical battle of wills. Violence is meant to be her salvation, but the bruised look she sends out suggests a different, and more interesting, story. Her glare tells us that what saves her is also what immolates her.