Depends on the game don't it?
Mass Effect I see it as a character, I'll play the role of the type of person it creates from it's bio. A survivor who stood the line to defend a colony? She'll be a military commander who fights for her people and her planet and later citadel for what will be best for the role of the many.
Kain from Legacy of Kain? He's evil, I play him evil. Silly ripped people's skin off with a blast.
An angel fallen to Earth? Well he's good and will do what he can to help all others.
Good or bit of a jerk and still good Mickey from his Epic story? I'll be good for Mickey's sake.
Neutral like Cole of Infamous? I'll let the game design for me with what would be the best and most effective choice to help out the character and his friends. If it's good, I'll be good, if it's evil, I'll go for evil.
If it's just me, a character I roll myself or build my own start and choices, say Fallout 3? It's a me, McAster. I'll play and respond how I would if I was in the same situation and issues. Which I tend to think as doing the right thing and doing unto others as they have done unto me save for very good reasons.
It's all about the story for me. I want to get the most out of it with the character, so I'll pick what I feel will best deliver that. Of course I'll then play the games over again just to see what I missed with the other alignments.