Ted Hope: voice on answering machine from Late Show with David Letterman (1993).
When Harvey Pekar first guest stars on Late Night with David Letterman (1982), Joyce is in the waiting room with a man with an alligator (Terrence Sullivan). This was originally intended to be a cameo by John Waters.
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.
Gary Dumm: longtime American Splendor artist is the gray-haired man getting his copy of "Our Cancer Year" autographed.
The jacket that Paul Giamatti wears for Harvey's first Late Night with David Letterman (1982) appearance is the exact same piece of clothing that we see the real Harvey Pekar wearing on the TV.