Jason Todd: played by Ben Foster, Jason is the psychopathic ex-Robin who's acting pretty lively for a dead man.
Dick Grayson: played by Matt Bomer, Dick has grown out of the short pants and into Nightwing, helping his mentor Batman in the fight against Gotham's united army of supervillains.
Harley Quinn: played by Kristen Bell, Harley is under the thumb of Hush and the Riddler. Not that she minds shaking up trouble, flirting with villains, and robbing operas.
Batman: played by Christian Bale (because in my head continuity THIS should be the final Nolan film), Batman is fighting a baffling war against most of Gotham's most notorious villains, not to mention one against someone he considers a friend.
Catwoman: played by Keira Knightley, Selina Kyle's dangerous alter ego is Gotham's most famous burglar--as well as the love interest of the Caped Crusader.
Thomas Elliott/Hush: played by Ewan McGregor, Thomas Elliott is a surgeon and the childhood friend of Bruce Wayne...who harbors a dangerous grudge.
Poison Ivy: played by Rachel-Hurd Wood (even in my head I require actor continuity within a franchise's movies), Poison Ivy is just one of Gotham's supervillains working with the Riddler and Hush against Batman.
The Riddler: played by Sam Rockwell, The Riddler joins with Thomas Elliott to take down Bruce Wayne and Batman.
Tim Drake: played by Skandar Keynes, Tim is the current Robin. He is kidnapped by a costumed figure who seems to be the late Jason Todd.
Alfred Pennyworth: played by Jude Law, Alfred is Bruce Wayne's long-suffering and completely badass butler.
And that's how Diana casts it! All images pulled from Google and Wikipedia.