If you're not fully up to speed with George R.R. Martin's books and are looking to avoid any spoilers, you might want to stop reading now...
Given that A Storm of Swords culminates with the Battle of Castle Black - a showdown between Mance Rayder's (Ciaran Hinds) Wildling army and the men of the Night's Watch, supported by Stannis Baratheon (Stephen Dillane) - it's not surprising that Marshall has been asked to return, seeing as the action-packed 'Blackwater' was the stand-out episode of Season Two. According to Empire's source, the director is already prepping the finale, and has been afforded a much longer lead-in time than he was with 'Blackwater', meaning we should be in for a treat when the episode airs next year.
Season Four of Game of Thrones is set to go into production later this year. So far, the only confirmed addition to the ensemble cast is Pedro Pascal (Graceland), who is set to portray Oberyn Martell, a.k.a. The Red Viper.