Something great about John Patrick Lowrie and Mary Kae Irvin's performances as citizens in Half-Life 2 is that they make the citizens sound both extremely generic, but also extremely expressive. The voice of just an average everyday person, yet it doesn't feel like citizens are talking to themselves, but to each other with how many different expressions and tones are done with the voices. I don't think Valve could've chosen anyone better.
(Edited)
If you review EP1 and EP2 VO files for the Citizens, They are really expressive and kinda angry and violent, Here are many lines under Their leader's gone! Why the hell are they sticking around? *For soldiers* Damn these things! *For Antlions* Don't let that thing get ahold of your head! *For Headcrabs* Oh, hey Freeman. And this must be the little lady. *For Alyx* Look out, it's gonna unload! *For Gunship*