I've composed and produced video game scores in the past, as well provided some voicing for characters here and there for apps and indie games. It's one of the more fun kind of projects I work on, but I haven't been anything close to a 'gamer' since Nintendo was "super" and character renderings were two-dimensional, so my interpretations of what's needed are sometimes, just that: interpretations.
But nothing beats speaking to a gamer to realize the real necessities of the craft. Even the game developer can have a limited amount of real input, because even though most of them are gamers themselves, this is their "baby". They're very close to the characters and vision they have in their head, so sometimes, like all of us who create things for a livelihood, they get a little stuck on that vision and don't hear the awesome things you might be able to bring to it that they haven't thought of.
I was lucky enough recently to get some really nitty-gritty input from a gamer that helped not only me, but maybe you! Here's some highlights for voicing video game characters with the player in mind:
1. Make the 'gift of gab' tolerable! Unlike audiences for other kinds of voice acting, most gamers spend long spans playing at a time, so make sure your voicing won't be grating if listened to for hours! If it actually makes a player take breaks just to get away from your voice...you're doing it wrong.
2. Don't Overdo it! A lot of us may feel the 'over the top' characters in video games need 'over the top' voicing to match. You may be playing a super-being, but you don't necessarily have to voice them that way. One of the benefits of having a player listen to you for hours on end is that they really get a 'feel' for the character in a way other mediums don't allow for, so relax a little. This isn't a 30-second spot for "Super Tax-Man"! A lot can be conveyed in subtlety, and the player will appreciate that!
3. Stop, look, and listen! If you're going into game voicing blind, there's no better time to study. Talking to gamers is a step in the right direction, as is watching them play, seeing how they respond to other games with voicing already on the market. Watch anime, and see how the dialogue plays. Research the actors at the top of the field in both gaming and anime, and see what their techniques are. It won't be hard to find them - just ask a gamer! The biggest names get fans of their own, and have online accounts they're promoting on.
If you're afraid you're going to have to become a gamer to voice games well, though it would help, it's not necessary. There are resources all around you, from online forums for gaming, to YouTube, to your friends and family members who love the games. Put them to use, and get to work!