The MCU has been for a long time. So it's natural not to remember every actor that ever appeared in the franchise. That said, there are some very famous ones that we have somehow managed to forget. 

1. Game of Thrones star Natalie Dormer appeared in Captain America: The First Avenger as Private Lorraine, who stole a kiss from Steve Rogers. 

Source: Insider

2. Harry Potter & Game of Thrones actor David Bradley played the church keeper guarding the Tessaract on Norway on Captain America: The First Avenger

Source: WhatCulture

3. Actor Nathan Fillion made a brief cameo as an inmate in Guardians of the Galaxy.

Source: Screenrant

4. Modern Family's Phil Dunphy, aka Ty Burell played the role of Leonard Samson, Betty Ross' boyfriend from The Incredible Hulk

Source: MCU Wiki

5. The Hobbit star Richard Armitage played Heinz Kruger, the HYDRA assassin that killed Abraham Erskine, in Captain America: The First Avenger.

Source: Comicbook,com

6. The Walking Dead star Ross Marquand appeared as Red Skull in Avengers: Infinity War and Endgame

Source: Comicbook.com

7. Shazam actor Zachary Levi played the role of Fandral in the Thor movies. 

Source: Screenrant

8. Following in the footsteps of her husband, Paul Bettany, Oscar winning actor Jennifer Connelly also voiced an AI interface in the MCU. She was the voice of Karen in Spider-Man: Homecoming

Source: The Jakarta Post

9. It can be difficult to recognise him without his glorious hair, but Jeff Bridges was part of the cast that kickstarted the MCU. He played Obadiah Stane in Iron Man

Source: Slash Film

10. Given that Iron Man 2 was one of the weaker films in the MCU, it is easy to forget that legendary actor Mickey Rourke actually played the bad guy, Whiplash, in that film. 

Source: Herald Extra

11. Before Crazy Rich Asians, Michelle Yeoh appeared in Guardians of the Galaxy Vol. 2 as Aleta Ogord. She has also been cast in Shang-Chi and the Legend of the Ten Rings

Source: Syfy

12. Ken Jeong appeared as the security guard who helped Scott Lang after he came out of the Quantum Realm in Endgame

Source: Reddit

13. Yvette Nicole Brown appeared in a cameo role in Endgame. She was in the lift at SHIELD when Steve Rogers went there to steal the Space Stone. 

Source: Screenrant

14. Jim Rash, the Dean from Community appeared as the M.I.T Liason who spoke to Tony Stark about his donation in Captain America: Civil War

Source: Reddit

15. Donald Glover played the role of Aaron Davis, the uncle of Miles Davis in Spider-Man: Homecoming

Source: Screenrant

16. In the MCU, Danny Pudi appeared in Captain America: The Winter Soldier as Moore, a S.H.I.E.L.D. communications officer.

Source: Bad Taste

17. Sylvester Stallone also appeared as Starhawk in Guardians of the Galaxy Vol. 2

Source: Reddit

18. Doctor Who's Jenna Coleman also appeared in MCU as Bucky's date in Captain America: The First Avenger

Source: Business Insider

19. Olivia Munn is already a part of the X-Men Universe. But before she landed that role, she appeared as a reporter, Chess Roberts in Iron-Man 2

Source: The Things

20. Sons of Anarchy alumni, Walton Goggins has also done a cameo in the MCU. He appeared as a lower-level criminal in Ant-Man and the Wasp that tried to sell Pym's technology on the black market.

Source: Slash Film

Actually, if you start looking for it, there are tens of other famous actors who you won't even be able place in the MCU!