Obviously spoilers ahead, if you haven't read A Storm of Swords...
Martin was at ConQuesT in Kansas City this past weekend, and he told the convention-goers that auditions are being held for season four, including at least three new characters:
The Magnar of Thenn. Styr isa major part of Mance Rayder's Wildling army — although a lot of his part in the book has already been given to Tormund Giantsbane. In the book, he leads the scouting party that climbs the wall, which Jon Snow and Ygritte are part of. But presumably he'll play some other huge part in the Wildling attack on Castle Black.
Mace Tyrell. The son of Lady Olenna and father of Margaery and Loras — we've already heard from Lady Olenna that he's a disappointment. Presumably coming to town for Margaery's wedding.
Oberyn Martell. The first person from Dorne we've really met — he comes to King's Landing to take a seat on the Small Council, and winds up playing a huge role in Tyrion's storyline.
None of these inclusions are terribly surprising, although you could have imagined Styr being left out at this point.
Also, someone in the audience asked Martin what Podrick did — and at first Martin, who'd just been talking about the Battle of Blackwater, looked confused and said, "He stabbed him with a spear." Then the questioner clarified that he was asking what Podrick did in a more recent episode, to which Martin's response was still: "he stabbed 'em with a spear."