Via a new post at “Not a Blog”, GRRM provides hints for a supporting role that has been cast. The post, titled “The Old Knight”, must refer to Ser Rodrik Cassel, the master-at-arms of Winterfell.
GRRM writes as follows:
... is an experienced master at arms,
... has been a colonel, a doctor, a friar, a nazi, a sergeant, a superintendant, a corporal, and a minister,
... knows Cersei, Eddard, Ser Jorah, and Pussy Galore.