Video games are almost never known for good acting. Voice-overs and cutscenes tend to be stilted, and use lesser-known and, often, lesser-skilled actors to do the dirty work. Occasionally you can find a good actor or two hired to do scenes, like Patrick Stewart in The Elder Scrolls: Oblivion, Vin Diesel in The Chronicles of Riddick: Escape from Butcher Bay, or Mark Hamill in Wing Commander 3 and 4, but usually you can expect mediocre voicework and cutscenes at best, bumpers to click through in between gameplay.
Then I started playing Command and Conquer 3: Tiberium Wars. The Command and Conquer series has always had well-produced, amusing cutscenes that were worth a watch despite the inevitable camp. The cutscenes in Tiberium Wars are not only well-produced, but laden with a surprising number of sci-fi stars and character actors. It's pretty much an orgy of nerdish cinematic delight.
Battlestar Galactica Cylon babes Grace Park and Tricia Helfer are two of the most notable actors in the game, playing GDI offier Sandra Telfair and NOD general Kilian Qatar. Josh Holloway, Lost's Sawyer, plays NOD's agent Ajay. Billy Dee Williams (Star Wars' Lando Calrissian, and if you didn't already know that then get off my lawn) plays GDI director Redmond Boyle. Finally, Michael Ironside plays GDI General Jack Granger. You might remember his gravely voice as that of Sam Fisher's in the Splinter Cell series and Darkseid's in the Superman and Justice League animated series. He also plays nearly every other crusty, war-hardened soldier in any show or movie that needs one.
Of course, it's not all Hollywood (or Vancouver, or wherever they shoot Battlestar Galactica and Lost) stars in C&C 3. Joseph Kucan reprises his role as the bald, sinister, evil mastermind Kane, mostly because fans of the series would start rioting if EA Games did otherwise. Kucan IS Kane. The Brotherhood of NOD's leader couldn't be played by anyone else. Courtenay Taylor also lends presence as the alien Scrin A.I. You might not know her name, but you probably know her voice from Supreme Commander, Knights of the Old Republic, Call of Duty 2, and God of War. She played the twins in the game's...controversial mini-game.
If you're a fan of Battlestar Galactica or Lost, or just like campy sci-fi scenes with some pretty big names, Command and Conquer 3 is a great pick. Of course, the game itself is darn good, too (it scored a 9.0 and an Editor's Choice from GameSpot). If it wasn't, all the actors in the world wouldn't help it.