Joss Whedon says he’s concerned that Mike Birbiglia’s Sleepwalk With Me, which opens August 24 via IFC, is “creeping up” on Marvel’s The Avengers and might eclipse the superhero movie’s dwindling weekend take when it debuts. The $1.46B global grosser Avengers took in about $271K from 220 theaters last weekend. Produced in conjunction with the radio show This American Life, Birbiglia’s movie is based on his sleepwalking and life misadventures and one-man show. His movie was a favorite this year at Sundance and South By Southwest. Fan fervor, Whedon notes, has resulted in the doubling to 80 the number of theaters opening Sleepwalk With Me. Whedon says, “That makes me nervous…”: