
Completely, hypothetically speaking. This sort of question has been floating around game forums all over the place for ages now. I do sort of hope that they never, ever make a film out of the Half-Life series because there's a pretty good chance that it would be entirely awful. If it was decided to make Gordon speak, many fans would be outraged. I wouldn't, but I've never liked the mute protagonist thing. If he didn't speak, it'd be immensely difficult to make it reasonably work without boring audiences to tears. Oh, this is, of course, totally for Half-Life 2; it has a better story. Here is the all-star cast!
Gordon Freeman - Hugh Laurie

Pretty good resemblence if you stuck nerd glasses on his face. He's a good actor for a Cockney weasel, and big could even probably pull off the necessary expressive facial features for the mute permutation of Freeman. Honorable mention goes to this guy. He's obviously a big science nerd, but can he act?
Alyx Vance - Rosario Dawson

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Not quite as striking a resemblence, but Dawson has proven that she has a pretty decent ability to act; she can be both gritty and sensual enough to pull off that all sides of Alyx's character.
Dr. Eli Vance - Morgan Freeman
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This one's just too obvious.
Dr. Isaac Kleiner - Ed Harris
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Pretty good resemblence, and Ed Harris definitely knows how to act. The only concern I would have would be Kleiner's voice not being quite right from Harris, but i still think he'd do an excellent job in the role. David Cross would also be a decent choice, but he seems often too absurdist or silly to fit into the semi post-apocalyptic Orwellian society.
Barney Calhoun - Matthew Broderick


Another pretty solid resemblence. Matthew Broderick tends a lot towards the comedic side of life, which would make him great for the wise-cracking sarcastic side of Barney. Whether or not he could pull off the grittier side of it is anyone's guess, but I think he could manage it.
Dr. Wallace Breen - Ian Holm


Yet another pretty good resemblence. Holm is most definitely versitile enough to pull off a spectacular Dr. Breen, and it'd be great to see him in the villain role. Other people have suggested Sean Connery or Patrick Stewart, but I think that either of those two would provide too much of a dominant presence in that spot.
Father Grigori - Peter Stormare


Ah, where would we be without the slightly psychotic preacher and his flock of mindless headcrab zombies? Stormare can do and has done the off-balanced role a few times, and he definitely has the voice to be able to handle it. Another fun choice for this would be Robin Williams (who can pull off just about anything), who is a big fan of the series and was actually slated to do voicework for it, but had scheduling conflict.
The G-Man - Christopher Walken


He'd be perfect for it. He could easily put out that halting, strange speech of the G-man while looking incredibly creepy and sinister.
Well, in any case, even though it's completely hypothetical, it's still a fun thing to think about. Half-Life 2 has an absolutely stellar story, and if they attempted to not vomit the usual Hollywood crap all over, it could make a pretty darn good movie.