But the movie's title is "Greenberg, " not "Marr, " and Mr. Stiller commands a kind of awe in his refusal to clothe the naked hostility of a character who suffers from the autoimmune disease of self-loathing. ("Life is wasted on people, " the hero says at one point.) An ardent arsonist, he has burned most of the bridges that once connected him to friends and a flourishing career.