No, seriously. We're actually not kidding. It's J.K. Simmons, who scored an Academy Award for his role in Whiplash. The beloved badass and Law & Order vet pops up as a guest-star in Thursday's episode of the NBC cop comedy as Detective Dillman, a former colleague of Capt. Holt (Andre Braugher) at the NYPD. Dillman enters the picture after a messy prank is committed in the precinct and goes terribly awry, prompting Holt to lean on his old friend — the best detective he has ever known — to assist in this Whodunnit? Nope, that's not the right phrase. As Holt explains to Jake (Andy Samberg) in this exclusive peek at the episode, it's the more grammatically acceptable Whohasdonethis?