It feels like a sweeps moment -- a big name star guesting on a long-running show. But it's not set for sweeps. It is, however, a special occasion and a reason to do something festive. So, on April 29, Robin Williams will make a guest appearance on Law & Order: SVU in part to celebrate the 200th episode of the intense NBC crime drama.

The Oscar-winning actor (for Good Will Hunting) is no stranger to television, as we all know. Till the day he dies, he'll be Mork. Na-noo, na-noo. As an alien from the planet Ork, Robin Williams was catapulted from obscure comic actor to overnight sensation thanks to the Garry Marshall sitcom Mork & Mindy. It was the beginning of a stellar career.

But Robin Williams is also no stranger to drama. He studied drama at Julliard under John Houseman and performed Shakespeare as a young actor. On TV, he was also Emmy nominated for a dramatic turn on Homicide: Life on the Street.

For this L&O: SVU, he'll be play an engineer becomes an agent for radical change, challenging authority, when his peaceful life is turned upside down by tragedy. L&O: SVU executive producer Neal Baer describes Williams' storyline as "powerful and complex." Baer collaborated on the script with co-exec producer Amanda Green. "We can't imagine anyone else doing this role," said Baer.