Here's the difference between movies and television: At the film studios, they make movies based on old TV series, while at the broadcast networks, they make TV movies about the making of old TV series. NBC announced that it's following up its first two successful ''Behind the Camera'' movies (last year's tale about the making of ''Three's Company'' and last month's similar take on ''Charlie's Angels'') with four more backstage sagas drawn from rerun favorites. Soon to get the ''Behind the Camera'' treatment are three more ABC shows -- ''Laverne & Shirley,'' ''Mork & Mindy,'' and ''Bewitched'' -- and one NBC sitcom, ''Diff'rent Strokes.''
There's plenty of dramatic fodder in the on-the-set stories of these shows -- the rivalry between Penny ''Laverne'' Marshall and Cindy ''Shirley'' Williams, the shark-jumping switch from Dick York to Dick Sargent in the role of ''Bewitched'''s Darrin, the rise to stardom of ''Mork'''s Robin Williams, and especially the legal woes of the child stars of ''Strokes.'' Meanwhile, ABC is getting into the game with a movie about the creative catfights behind the network's classic nighttime soap ''Dynasty.'' If this keeps up, the ''Behind the Camera'' franchise could become so huge that, in 25 years, we'll see a series of ''Behind 'Behind the Camera''' movies.