Luna's main claim to fame in this film is helping Harry along in figuring out the identity and location of Voldemort's Hogwarts Horcrux. When Harry announces that he thinks the Horcrux has something to do with Ravenclaw, Luna chimes in pretty quickly:
HARRY: I think it has something to do with Ravenclaw. Um, it'll be small, easily concealed. Anyone, any ideas?
LUNA: Well, there's Rowena Ravenclaw's lost diadem.
RON: Oh, bloody hell, here we go.
LUNA: Lost diadem of Ravenclaw? Hasn't anyone heard of it? It's quite famous.
CHO: Yes, but Luna, it's lost. For centuries now. There isn't a person alive today who's seen it.
Despite Ron and Cho's naysaying, Harry ends up agreeing that they're probably looking for Ravenclaw's lost diadem and goes in search of it in Ravenclaw Tower. Once again, though, Luna has to step in and give him a nudge in the right direction. She literally has to chase Harry down to get him to listen, though:
LUNA: Harry, wait! I need to talk to you!
HARRY: I'm a bit preoccupied at the moment.
LUNA: Harry Potter, you listen to me right now! Do you remember what Cho said about Rowena Ravenclaw's diadem? There is not a person alive who's seen it. It's obvious, isn't it? We have to talk to someone who's dead.
Thanks to Luna, Harry finds out the location of the diadem from Ravenclaw's resident ghost, the Grey Lady (a.k.a. Rowena Ravenclaw's daughter). Luna's super sweet, but we're glad she can be insistent and tough when it matters.