and still do the majority of the quest? No spoilers please.

Maybe not do the majority of the quests, but a rather large number of them.

I can only think of one point in the main quest line where you might be unable to avoid a fight. But then that's more someone else picking a fight with you than you picking it with them.

Yeah, its possible. But its boring IMO.

In this instance I'm gonna have to say from what I can tell, no. Pretty sure no matter what faction you side with you're gonna have to kill someone eventually. There are alot of speech options though and you can talk your way out of alot of things in this game.

If you use beanbag rounds and boxing gloves probably yes.

http://www.neogaf.com/forum/showpost.php?p=24221124&postcount=9108

(http://fallout.wikia.com/wiki/User_blog:Ausir/Fallout_tidbits_15-2010)

Yes. But not necessarily a pacifist in the strictest sense of the word. You might have to knock people unconscious at time - which can be done with boxing gloves or shotgun beanbag rounds.

You can beat the game without firing a shot! but you'll need to manage your char like crazy. You'll need high sneak, maxed out speech and an unlimited supply of stealth boys. You'll also need to ignore about 60% of the quests.

Well, there are several quests where you have no choice but to kill. However, there are others where you can find ways to talk around it.

If you were planning on doing a pacifist run, you should probably consider having companions (they can do the killing for you), and have a very high speech and barter skill set. Maybe throw in a large amount into perception and sneak, if you want to move past enemies without engaging them.

Pacifist in the sense that you do not join a faction and do not kill any other humans, that's fine. I can about understand how that would be possible.

But not killing any creatures? That would be difficult, although the main roads do protect you from combat and allow you easy access to every location. But even so, many quests would have to be missed unless you recruited a companion to do your dirty work for you.