Awesome, Dear Boy: Some of the cast of Sin City signed on after seeing the test footage which would eventually become the opening scene. In the making-of it's noted that a lot of them had wanted to play Film Noir characters for a long time, such as Bruce Willis. Also, many cast members signed on due to other actors being in talks. Considering the large cast of famous faces, this seems reasonable.
In the past Frank Miller has promised several new stories, including a Hartigan prequel, a story set in 1940, and a story set in the Sin City's founding during the Fold Rush. So far, the only new stories written are the ones being featured in Sin City 2.
It is currently unclear whether the role of Shelly will be recast because of Brittany Murphy's untimely death, replaced with a new character or written out entirely, since she has a fairly important role. Given the trend of simply recasting parts, it seems likely a new actress will be hired.
Type Casting: Mickey Rourke as an ugly, down on his luck bruiser with a heart of gold? He was born to play Marv. In fact, when Frank Miller met Rourke, the only thing he wrote down in his notes was, "Met Mickey Rourke — HE IS MARV."