heralde is right: Northern Calloway was, unfortunately, quite mentally unstable, the only reason he was still on the show was similar to Carlo's fear, to avoid having any kind of public humiliation that "Sesame Street's 'David' in trouble", etc, and they made sure he had like a supervisor, or a "guardian" on the set with him at all times while they shot. But actually, he was still on the show up until his death: he was there when Gabi was born in 1989, and that's the same year of his death; they explained on the show that David moved in with his grandmother, who needed help running her farm, and that David sold Hooper's Store to Mr. Handford.
I've heard similar stories regarding people not particularly like a show they worked on: there was a lot of controversy from the beginning surrounding Hogan's Heroes. The pilot episode included a Russian POW, but when the series was picked up, he refused to sign on because of how the Nazis were made to look like lovable buffoons; conversely, Werner Klemperer only agreed to play Colonel Klink so long as he WAS made to look like a fool. A lot of the actors who played the Nazis were, in fact, Jews who fled Hitler's Germany; John Banner, who played Sergeant Schultz, even said in one interview, "Who better to play the Nazis than we Jews?" There was some initial friction between Bob Crane and Richard Dawson, since Dawson wanted the part of Hogan, but Hogan HAD to be the ALL-AMERICAN war hero, and his accent was still to heavy at that time, though in recent years, Dawson has mellowed down, and gotten his ego in check, and has said despite the "problems" on the set, he is glad he had the opportunity to be on such a long-running show.
But getting back to ALF, you'd be surprised, even though there are a lot of people who would love with work alongside a puppet like that, there are just as many who would rather now, and it's kind of sad. In Caroll Spinney's memoir, he writes about a rapper who was brought onto the set in the 70s to do a rapping alphabet with Big Bird, but he kept flubbing the lyrics... FINALLY, he got it right... then Caroll flubbed a line at the end, and when they said they'd have to do yet another take, the rapper got physical with Big Bird for ruining the shot, and smacked the puppet's head right out of Caroll's hand, and it fell to the ground. Sadly, shows like these, the puppet characters ARE the stars, and the actors have to learn to accept that, BUT, the actors' egos get the better of them, and they can't accept that this fake construction of foam, fleece, fur, etc is the "star", and not them.