Kilmer, who plays a faded rocker in Terrence Malick's untitled new film, hit the stage in character, thrashing about, screaming at the crowd and smashing a guitar. He also hacked off his hair with a penknife.
He was joined by co-stars Fassbender and Mara, who shared a kiss at the side of the stage.
Mara played guitar, while Fassbender stood in the wings for Kilmer's odd performance, which was reportedly filmed for director Malick's movie. The film also stars Ryan Gosling, Cate Blanchett, Natalie Portman and Christian Bale.