Obsidian's creative director Chris Avellone has been answering some fan questions related to Planescape: Torment on his blog, mostly focused on the choice to put the player in the shoes of the mysterious Nameless One:
In many RPGs, including the Fallout games you've worked on and the tabletop games that are Planescape: Torment's genesis, the design seems oriented towards giving the player a (blank slate) to play as. But PS: T has the player inhabit a fairly developed character. Why did the team choose The Nameless One as the main character?
So it was a juggling act - how do you give a player as much freedom to create a character while knowing you don't have the art resources to create a lot of customization for that character? We purposely chose a single template and then used the narrative of amnesia and the curse to explain the role-playing range the character could have (each time you wake up, your personality has a chance to shape itself in new directions based on the player's experience) and also changed the "armor/costume accumulation" in most RPGs to gathering tattoos and other items that wouldn't drastically change the character model. Note that many of the companions had similar limitations, accounting for their largely static appearance throughout the game.
Note that if I'd had the resources, we would have had a much larger range of character customization options. It wasn't our desire to limit it to a single character look, and while it did work, at the same time, it didn't give you as much freedom as we would have liked.
My research is focused on addressing the relations between players and the characters they (control). What sort of relation (or relationship) did you want to see in Planescape: Torment between the player and The Nameless One?
The Nameless One is an avatar that allows you to view Planescape through "ignorant eyes." It was the first exposure most game players had to the universe, and we wanted the Nameless One to be a brand-new vessel that would explore that world with the player. While amnesia is a tired clichÃ©, we felt it was a great mechanic here to reinforce the wonder of the universe. In addition, pragmatically, there's simply too much of Planescape to assume a player has knowledge of it all - giving them a blank slate was the best option, in my opinion.
Lastly, we wanted the player to feel the weight of their own history, and feel a desire to know what these previous incarnations had done, and realize that much of the world they found themselves in had been shaped by actions they'd done in the past by someone who'd been a different personality in their shell. The entire game is very selfish and egocentric, and I felt strongly that's one of the best things to provide for a player.