[@Happy Go Lucky]

Basically, you create a Player (e.g. Joe Smith) who is participating in the virtual world. He/She has been put in the body of an Avatar of their choosing (e.g. MasterChief) and has the abilities of said Avatar (Regen Health, Vehicle Flipping etc.)

Now, for the most part, your Player would be in control of the body, but he/she may inherit personality traits of the Avatar. How your Player's personality and your Avatar's personality mesh is up to you.

For example:
[url=http://www.roleplayerguild.com/posts/2936113]Nicholas[/url] is cynical, lazy and apathetic, but in the body of Gaige, he became a little friendlier.

[url=http://www.roleplayerguild.com/posts/2936439]Alton[/url] tries to be emotionless, but in the body of Jacket, became incredibly prone to rage.

Hope this helps!