According to Entertainment Weekly, Þór “Jónsi” Birgisson, Georg Hólm and Orri Páll Dýrason are currently in Croatia shooting a cameo appearance for the series, although the site says it is "sworn to secrecy" on the nature of the band's involvement.
Game of Thrones season four is due to begin on HBO in the Spring, with new additions to the cast including Mark Gatiss (Sherlock) as Tycho Nestoris, Joseph Gatt (Banshee) as Styr, Roger Ashton-Griffiths (Gangs of New York) as Mace Tyrell, Pedro Pascal (Graceland) as Oberyn Martell, Indira Varma (Rome) as Ellaria Sand, Michiel Huisman (Nashville) as Daario Naharis, Dean Charles-Chapman (The White Queen) as Tommen Baratheon and Icelandic strongman Hafþór Júlíus Björnsson as Gregor 'The Mountain' Clegane.