The most exciting part of this news for me is Alexander, who gets to play the fierce warrior-goddess Sif. She was once a lass with golden hair (just like her "real" Norse counterpart) but Loki was jealous of her hair color, stole it, and replaced it with enchanted troll hair. Jerk. But Sif shrugged it off, picked up a big sword, and became one of the toughest warriors of Asgard. She also became Thor's lover, and while she accompanies other male Asgardians into battle, she also sticks by her Thunder Man. If you're wondering how he could leave such a kick-ass girl for a mortal like Jane Foster (who will be played by Portman), don't think too badly of Thor. Once he went mortal as Donald Blake, he became confused and convinced Jane was the reincarnation of Sif. Reportedly, Donald Blake isn't in the Thor movie, so perhaps they're just going with a standard love triangle, and Thor will have to chose between his mortal and (relatively) immortal girlfriends.
Feore's role is being kept a secret, and Marvel will only say that he's playing a villain. I'm going to just take a wild guess that he's going to play Mephisto, and that's just because of Sif and her love connection. But perhaps he's playing Surtur so they have someone to help Loki bring around Ragnarok. Your guess is as good as mine and since THR is reporting that not even the actors have seen the script, guesses are all we have to go on.