Concurrent with his appearances on "The Addams Family", Cassidy began doing character voices on a recurring basis for the Hanna-Barbera Studios, culminating in the role of Frankenstein Jr., in "Frankenstein Jr. and The Impossibles" series. He was the voice of "Meteor Man" in "Birdman and the Galaxy Trio", as well as the hero in the Chuck Menville pixillated short film "Blaze Glory", in which his already-deep voice was enhanced with reverb echo to give the character an exaggerated super-hero sound. Cassidy also voiced Ben Grimm ("The Thing") in "The New Fantastic Four". Cassidy went on to perform the roars and growls for Godzilla in the 1979 cartoon series that Hanna Barbera co-produced with Toho, and was also the voice of Montaro in the "Jana of the Jungle" segments that accompanied "Godzilla" during its first network run. His voice was the basis for the sinister voice of Black Manta, as well as Brainiac and several others on "Super Friends". Cassidy was the original voice of Moltar and Metallus on "Space Ghost" from 1966 to 1968. Cassidy's final role was as King Thun of the Lion Men in the television animated feature film "". That particular role was originally recorded shortly before Cassidy's death in 1979, until the decision was made to use the footage for a television series, "The New Adventures of Flash Gordon". As such, Cassidy's death necessitated his role being recast for the series with Allan Melvin. After the series' conclusion, the original feature film and soundtrack were reassembled and broadcast in prime time in 1982 with Cassidy's performance used. After "The Addams Family", on the TV series "The Incredible Hulk", he provided narration of the title sequence, and the Hulk's growls and roars. In deleted scenes from the original "Battlestar Galactica" TV pilot movie, "Saga of a Star World", Cassidy can be heard providing temporary voice tracks of the Cylon Imperious Leader, before actor Patrick Macnee was contracted to voice the character.