8 Celebrities You Didn't Know Were on Will & Grace

Will & Grace had some unforgettable guest stars during its first eight seasons on the air. Cher, Madonna, Britney Spears, hello? It doesn't get much better than that.

Though there were tons of big names that hung out with Will (Eric McCormack), Grace (Debra Messing), Karen (Megan Mullally), and Jack (Sean Hayes), there were some major celebs on the show that we didn't even know would one day become mega-famous. You know, it was before they had their big breaks and all?

Knowing these famous faces were on Will & Grace is going to make you do a double take.

If you're nervous about going to the doctor, as Karen is in one Season 6 episode, having him played by Jack Black may make you feel better. On second thought, if you've seen what happens in the episode, maybe not.

We'll never get enough of seeing Will Arnett dance, and thankfully, the funny guy brought his smooth moves to Will & Grace as Jack's fellow backup dancer for Janet Jackson.

Before John Slattery played the iconic ad man Roger Sterling on Mad Men, he played Will's brother Sam with slightly less gray hair.

After Show: Why Debra Wasn’t a Fan of Jeremy Piven

Find out why Messing "wasn't a fan" of Jeremy Piven when he appeared on the show in the clip, above. 

Nick Offerman played a plumber and the object of Karen's affection in one episode of Will & Grace. And it seems as though life ended up imitating art since Mullally and Offerman would eventually get married in 2003.

Before Joel McHale was the leader of Greendale gang on Community, he played one of Grace's dates early on in his acting career.

Look, it's Luke from Gilmore Girls! Without a hat on!

Clark Gregg once played one of Jack's boyfriends on Will & Grace. We can only imagine that was a little more stressful than keeping things together at S.H.I.E.L.D. as Phil Coulson.

So who's going to make a cameo this season of Will & Grace? Keep watching Thursdays at 9/8c on NBC to find out.