Who was up for the role of Joe?
According to Valerie Harper (in her new memoir "I, Rhoda"), there were a number of actors she tested with for the role of 'Joe' in 1974...
William DeVane (said he was a great kisser)
Michael DeLano (he would later play 'Johnny Venture')
Carmine Caridiri (he would later play Phyllis' boss on her own show)
Richard Schaal (Harper's real life husband, but the producers had a rule about not having spouses act together)
Alex Rocco (who was hot off the success of 'Moe Green' in 'The Godfather')
It looked like it was going to be Rocco, when it was narrowed down between him and Groh, since Groh was a dramatic actor and not a comedic actor (and had trouble doing comedy). But there was a spark between Groh and Harper that the producers couldn't deny, and the writers realized Groh could be the 'foil'
to the other comedic actors cast around him. Groh got the role.
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