Well, it took several months to confirm Brendon's original exclusive scoop that Elizabeth Olsen would play Scarlet Witch in The Avengers: Age of Ultron, and indeed, things could have gone wrong in negotiations even if Joss Whedon had been interested in casting her for a long while, but now it's finally done.
Marvel confirms both Olsen and Aaron Johnson are now official cast members of The Avengers: Age of Ultron, which will begin filming early next year. Interestingly, even Marvel's own report refers to their characters as Scarlet Witch and Quicksilver, respectively, even though we had assumed they wouldn't even get to use those names due to legal technicalities with who owns the rights between Marvel and Fox.
Well, whatever they will be called in the film (my guess is probably just Wanda and Pietro), they'll be twins with superpowers who join up with Robert Downey, Jr, Chris Hemsworth, Mark Ruffalo, Chris Evans, Scarlett Johansson and Jeremy Renner to fight off James Spader's Ultron. I think this is great casting (yes, even better than Saoirse Ronan) so May 1, 2015 can't come soon enough.