Playing dark, Robin Williams has developed the burly insecurity and gargoyle frown of damned Edward G. Robinson in Fritz Lang’s Scarlet Street. With World’s Greatest Dad, he almost has the movie to match. Lance (Williams) is an unpublished serial novelist and an unpopular poetry teacher at the same high school attended by the son, Kyle (Daryl Sabara), he is raising alone, who incarnates every nightmare of downloaded premature debauchery, a physical virgin and Caligula of the mind. Sabara, whose few scenes of abject irredeemability leave a lingering stain on the movie, is point man for a perfect-fit cast, with standouts including his lone, school friend, Evan Martin, and Henry Simmons as an alpha-male teacher. The particular stew of midlife and pubescent despair that clogs a single-father male-child household has rarely been achieved so well: Lance’s parental tough love is a trailed-off “Ahh . . .” The details of leftover dinosaur-themed wallpaper (unnoticed as it peels) and the dirty sneaker prints on the glove compartment are enough to convince that screenwriter-director Bobcat Goldthwait knows his stuff. His cringer lands improbably on its feet after every reckless plot turn—involving autoerotic asphyxiation and fraudulent authorship—all the way until an over-fondness for music montage fells it in the last reel.