NBC would not lease out the actual Late Show with David Letterman (1993) footage where Harvey Pekar finally lashed out at David Letterman, so the scene had to be recreated with actors.
The jacket that Paul Giamatti wears for Harvey's first Late Show with David Letterman (1993) appearance is the exact same piece of clothing that we see the real Harvey Pekar wearing on the TV.
All of the "white room" documentary scenes were filmed in one day.
Jonathan Demme had tried to adapt a film version of "American Splendor" in the 1980s, but the project never came to fruition as Demme wasn't an established director.
Non comic book fans, or modern comic book fans, may be puzzled by the fact that the kid trick-or-treating as Green Lantern in the opening scene is wearing a red costume. This is actually accurate to the depicted era, the 1950s: the kid is dressed as the Golden Age Green Lantern, Alan Scott, who had a costume like that, before the concept got rebooted in 1959 into the more familiar green uniform version.
Gary Dumm: longtime American Splendor artist is the gray-haired man getting his copy of "Our Cancer Year" autographed.