It's almost as if the writers worked on different TV shows, or knew each other, or something. You know they do, but a lot of small things go unnoticed until you read about them. But you probably noticed that detective John Munch has popped up in a lot of different TV series.
Richard Belzer’s sarcastic, conspiracy-loving Sergeant John Munch is the unlikely common thread among some of television’s most beloved series. The crossovers began with Munch—who began his TV career working as a detective on the David Simon-based Homicide: Life On The Street—making appearances on Law & Order, where not only is it revealed that the casts work the same universal beat, but Munch also discovers that he and Jerry Orbach’s Lennie Briscoe even shared a woman. After Homicide ended its run, Munch transitioned fully into Law & Order: SVU—though not seamlessly: Munch’s birthplace changed from Baltimore to New York and regulars from Homicide appeared as other, unrelated characters in SVU. The Munchiverse was stretched further when he popped up in Simon’s The Wire—a connection that raises questions like, “If Homicide’s Baltimore police were assisting Law & Order cops, were the Law And Order units also dealing with New York’s connections to the Baltimore drug trade of Marlo Stanfield et al.?”
Munch also was seen in The X-Files, Arrested Development, and was even mentioned on a UK series. And he's just one of the list of 21 at The A.V. Club. Link
Previously: Tommy Westphall's Mind