She's referring to how they speak (their 'voice') rather than their actual physical voice, if you see what I mean. ie not accents/whether they drop their 'h's but their attitudes/pov's/opinions. The problem might very well be, you're giving them the same opinion about some subject (probably your own opinion), whereas they need to have very differing opinions. This can be hard cos you might think, for instance, that smoking should be made completely illegal, so writing a character who actually smokes, is physically a difficult thing to do. Hope that's of some help. Maybe...
Basically, someone reading your script should be able to tell who is speaking without having to look at (or by covering up) the character names. If you get to that stage, you're alright.
Dan
Hello, Griff!
