Here I will include the actors who have appeared in adventure games. Their pages will tell you what game they acted in, who they played, and whether it was a live action or voice role.
Jack Black (Broken Age)
Andrew Chaikin (Bone series, multiple other games)
Gary Coleman (The Curse of Monkey Island)
Michael J. Fox (Back to the Future: The Game)
Dave Grossman (Codename Cygnus, Duke Grabowski)
William Kasten (Telltale's Sam & Max series)
DeForest Kelley (Star Trek: 25th Anniversary, Star Trek: Judgement Rights)
Nick Jameson (Indiana Jones and the Fate of Atlantis, multiple other games)
Christopher Lloyd (Toonstruck, Back to the Future: The Game, King's Quest)
June Lockhart (Tesla Effect)
Dave Madden (The Curse of Monkey Island)
Leonard Nimoy (Star Trek: 25th Anniversary, Star Trek: Judgement Rights)
Gary Owens (Space Quest 4, Space Quest 6)
Patrick Warbutron (Tales From The Borderlands)
Wil Wheaton (Broken Age)
Tom Wilson (Back to the Future: The Game - 30th Anniversary Edition)