They know their name, gender, race, specific appearances, preferences, fighting style, major skills, back story, personality, character quirks, etc.
Some people even know all that for three or four characters, or more.
I see posts such as, "My female Dunmer spellsword, Gandaelia, doesn't like vegetables, especially those harvested from central and northern Skyrim, where the soil is not as fertile and therefore the veggies aren't as savory, so this topic doesn't apply to her. My Imperial Scout, Gargand, however, enjoys cooking as a hobby, but avoids fish because of certain allergies he has to pink and gray salmon. He can tell the difference if he tries hard enough, but usually he doesn't risk it when he is out in the wilderness, for he knows he has to get back to his Breton wife, Mylia, in their 16 foot by 18 foot cabin 400 feet south outside of Winterhold."
Now, don't get me wrong. I enjoy immersion and roleplaying, when the game actually rolls around.
And I understand the need for something Skyrim-related to do.
But aren't you afraid that you are going to get the game, fire it up, start making your character and your character will just seem boring because you already know every little minute detail about him or her?
I eventually decided that I wasn't going to start planning a character or back story until a week or two before the game comes out, that way it's still fresh and exciting in my mind.