Crime. In law enforcement, everybody who goes through the police academy is aware of the grittiness. The person who seems least offensive is capable of an act which shocks the world to its core. Case in point — Dr. Marty Pino, the former guy in the medical examiner's office fired for having too much overtime. This is a serious understatement. Why would it be an issue for him to be at work for many hours? After all, death can happen anytime. To put it in a nutshell, Pino claimed more hours than actually worked in order to try and pay off gambling debts ("Point of No Return"). The house of cards quickly tumbled, and the aftermath sent Det. Mac Taylor (Gary Sinise) and his team reeling.
Even Det. Stella Bonasera (Melina Kanakaredes) understands not everyone is what he or she seems. Take her last boyfriends, for example. Frankie (Ed Quinn, Eureka) was a decent guy, matching Stella's passion with a talent for art design and appreciation. After he put a video of them having sex on the Internet, Stella dropped him like a hot potato. He retaliated by nearly killing her, but wound up dying by her gun ("All Access").
Drew (Kerr Smith, Dawson's Creek) was clever at the art of manipulation. He set Stella up in order to get revenge on Mac. When the final pieces of the puzzle were put together, Drew got hauled off to prison on charges of attempted murder ("The Thing About Heroes"). As far as anybody knows, he's still there.
While he never dated Stella, so is Shane Casey (Edward Furlong The Terminator). He plotted revenge against those whom he suspected of sending his brother to prison for a crime he didn't commit. Casey was wrong, and Det. Danny Messer (Carmine Giovinazzo) went to tell him the truth just before arresting him for being a serial killer ("Raising Shane").
Stella has the shock of her life, though, when a new case has a link to her. A call comes into the police tip line about a man dead in a theatre. This is hardly unusual. New York is well recognized for its theatrical presence, and it makes sense for it to be used as a crime scene in a particular episode.