There are two ways to do a musical series on television.
One is to set the musical aspect in a world where characters have a legitimate reason to spontaneously break into song. Shows like The Partridge Family, Fame, Glee, and Smash all took advantage of that trope, having their characters perform at any and every opportunity.
The other option is to have perfectly realistic settings, where you wouldn't expect anyone to suddenly commence crooning — only to have them do it, anyway. It's the ancient performance technique Broadway musicals have employed for years. After all, alley cats, homicidal English barbers, revolutionary French peasants, and your average historical figure, Ragtime-era or not, don't, as a rule, go around warbling their innermost fears, thoughts and feelings. It's a stylistic narrative choice.
Television shows that have chosen to follow the latter, more complicated path through the years have included Rags to Riches, Viva Laughlin, and, of course, that ultimate punchline, Cop Rock. (We're not counting special, one-off, fantasy musical episodes of otherwise non-musical programs, like Chicago Hope, Oz, Buffy The Vampire Slayer, That 70s Show, Scrubs, How I Met Your Mother, etc...)
There's probably a reason why all of the shows in the first category managed to eke out more than a single season. And the ones in the second category... didn't. (Technically, Rags to Riches was on the air for two years, but their first was a truncated, eight episode run, and the second was axed after twelve airings.)
That reason most likely has to do with a manageable suspension of disbelief. It's one thing to accept uncharacteristic singing coming from the stage, where everything is already artificial, non-realistic, and heightened. It's quite another to be watching a television episode that looks and feels exactly like Hill Street Blues (another Steve Bochco creation) only to suddenly see the hard-core cops and robbers start rapping or, even more disconcerting, belting out a heartfelt ballad.
To read about ABC's latest attempt to bring musical theater to America's living rooms, Galavant, which premiered this past Sunday, read my post at BlogHer Entertainment: http://www.blogher.com/galavant-maybe-not-worst-thing-ever-and-other-tv-musical-series-also-werent