I play as me, and make my decisions accordingly. Sadly, I expect the quest for achievements will force playthroughs where I'm making decisions i.a.w a plan, which I dislike doing. The Completionist from F2 springs to mind, as extremes of evil and good were required. Some examples from F2:
The warrants - I gave them to Derek, completely unaware of what the consequences might be, because that oily skip-rat baddie had tried to force my sister into being a hooker - **** him and his friends.
The Dark Seal - agonising choice for me. In the end, Elizabeth got old, because I thought at the time that I'd need to stay buff to defeat Lucien. I envisaged having to hack my way through the Spire to get to him...and was thus hugely peeved by the actual ending.:'(
Defending The Light - no-brainer, for me at least. I had a wife in the village, had danced and drank with the locals, and was the local bar-man and woodcutter. Grim was doomed from the moment that quest appeared.
I'll be playing F3 the same way the first time, exploring everywhere, and only advancing the main plot when I run out of other things to do. Can't wait!