The original concept for the show was different from what would come on the air. It would have been on ABC and set in an upscale Long Island town, not in Manhattan. There was no older brother, originally, and the housekeeper would have been younger and sexier. The production company balked at the changes the network suggested, and ABC lost interest.
Dana Plato was written out of the series when she became pregnant in 1984, and it was agreed that it would be out of character for the same to happen to Kimberly Drummond. However, she was allowed to return for guest appearances after the birth of her son.
When Conrad Bain signed on to do Maude (1972), the production company promised him his own show when that one ended. By then, 'Gary Coleman' was also signed to the company. When the planned updating of The Little Rascals (1955), which Coleman would have been in, was aborted, they paired them and decided to come up with a suitable premise.
When ABC picked up the show, the producers contacted Dixie Carter to return to play Maggie Drummond, but she was unavailable because shortly after NBC canceled, she had landed her role on Designing Women (1986), so Maggie was recast with Mary Ann Mobley.
Arnold's question "Watchoo talkin' 'bout, Willis?" was ranked #17 in TV Guide's list of "TV's 20 Top Catchphrases" (21-27 August 2005 issue). It was never intended as a catch phrase; the writers simply made it so after the positive reaction to Coleman's delivery in the premiere. Towards the end of the show's run Coleman had become so tired of it that he demanded it be retired; he has not uttered it publicly since his 1999 cameo as himself on The Simpsons (1989).
Mary Ann Mobley played Conrad Bain's love interest twice: first she played Arnold's teacher Ms Osbourne, who dates Mr Drummond in the 2nd season; then she replaced Dixie Carter as Maggie Drummond towards the end of the series.
In addition to the spin-off series "Facts of Life, The" (1979), characters from this show also made crossovers onto Hello, Larry (1979) and Silver Spoons (1982), thereby implying that all four shows exist in the same universe.
To help promote the series before its debut, Gary Coleman made appearances on The Tonight Show. NBC used clips of Coleman from those appearances as promotional spots announcing the coming of the series. The network primarily aired them during their coverage of the 1978 World Series.
Arnold's schoolyard bully Stewart The Gooch was frequently referenced to and spoken of, but was never actually seen in any episodes. In a least one episode however, The Gooch's voice was heard over the telephone.
Janet Jackson was in the early stages of her music career when playing the recurring role of Willis's girlfriend Charlene. Jackson did sing in one episode, and found it awkward to perform as such while filming the episode. Charlene was gradually phased out of the series as Jackson began to devote more time to furthering her musical career.
Dixie Carter was said to frequently clash with Gary Coleman on the set, and largely found her experience doing the show to be unpleasant. After leaving the show, Carter refused to talk about it while being interviewed.