As you can see, we have a lot of the main cast returning for the sequel. Marv, Gail, Wendy, and Goldie are all in A Dame to Kill For, which is set before the stories in the first Sin City movie (which is why Marv and Goldie are still alive). There is also going to be an original story about Nancy, which is where I am assuming that Hartigan and Senator Roark are going to come into play.
Returning Characters, New Actors
One thing that I hate about sequels is when they recast characters. They did that a lot for this movie, but for some of them, it really couldn't be helped.
Anyways, I think that Josh Brolin is a great choice for pre-surgery Dwight, and I was psyched when Robert Rodriguez put the above pic on his Twitter!
Although I am really disappointed that she is not being played by Angelina Jolie (see here), I think that Eva Green is a decent choice for Ava Lord.
Joey and Sally
Joey and Sally are two characters that appear at the beginning of the A Dame to Kill For story. Joey is a married man, and Sally is his go-to girl from Old Town who speaks in a babyish voice to please her customers (and ends up needing saving by Dwight). I think that Ray Liotta and Juno Temple are good choices, but since Temple is so young, it'll make it pretty creepy.
Johnny and Marcy
These are two completely original characters that have not appeared in any of the Sin City comics. According to Wikipedia, Johnny (Joseph Gordon-Levitt) is a cocky gambler, and Marcy (Julia Garner) is a stripper that he meets.
There is still no word on who is going to play Ava's husband, Damien Lord. Until I read that he was going to be playing Bob, this is who I thought that Jeremy Piven was going to play.