Fran's dad, Morty, was played (when you could fully see him) by Steve Lawrence - big time singer who you could often see on the Carol Burnett show. I would presume that since he was probably not as available (or affordable) as the script called for him to be, keeping him unseen was a good way around that.
Well I always thought it was kind of a running joke, they always talk about him but never see him. They do that on a lot of sitcoms, they give really vivid descriptions about a character who is not major but not minor frequently, to leave the viewer curious wanting more. I've seen it on Home Improvement, Hannah Montana, and I'm sure many other shows too.
posted over a year ago
They have. Eli David on NCIS was a mystery to fans for a while.