Do you do all the quest with one character; or with every character you make...
Or do you use different characters to do different quest lines?

There are a lot of quests.

Multiple characters

Depends on my character

Multiple characters do different things depending on what type of character i'm roleplaying.

Multiple.

Many toons. But usually 1 for being a completionist that uses my favorite skills/playstyle.

I always try to play with only one character but recently i've been doing different characters to have fun doing other things like alchemy that i don't use in the main characters.

I can't imagine one character that can do it all.

Many, many characters.

I first make a character (Of whatever race "goes with" the game) and do everything, because im impatient and want to experience the whole game in a decent amount of time. Then i make more characters who specialize, im on my 4th (total) at the moment.