In the pilot episode, among Fawkes' doctors are Drs. "Baker, McGann, Hartnell, and Troughton." These are the names of various actors who have played the Doctor on the British TV series Doctor Who (1963). A later in-joke along similar lines has Fawkes using a business card with the name "I.M. Forman" on it; this was the name of the owner of the junkyard that appears in the pilot episode of Doctor Who.
In the episode "Exposed," Darian releases a crazed-looking man from a secret experimental prison, and comments "Do I know you?" The man he lets out was played by Adam Storke, who was the lead in an earlier series Prey (1998) - where Vincent Ventresca played a doctor.
After the Invisible Man ended, Vincent Ventresca and Paul Ben Victor appeared together as gambling buddies in an episode of the NBC TV series Las Vegas. Their entertaining comedic chemistry was still intact and was acclaimed by fans.