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.
1. Jack Black
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.
2. Will Arnett
3. John Slattery
Before John Slattery played the iconic ad man Roger Sterling on Mad Men, he played Will's brother Sam with slightly less gray hair.
4. Jeremy Piven
Find out why Messing "wasn't a fan" of Jeremy Piven when he appeared on the show in the clip, above.
5. Nick Offerman
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.
6. Joel McHale
Before Joel McHale was the leader of Greendale gang on Community, he played one of Grace's dates early on in his acting career.
7. Scott Patterson
Look, it's Luke from Gilmore Girls! Without a hat on!
8. Clark Gregg
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.