Harry Potter's Freddie Stroma (Cormac McLaggen) just got cast in season six of Game of Thrones, as Dickon Tarly — Samwell’s brother.
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There’s not much known about Dickon from George R.R. Martin’s novels upon which Thrones is based. Dickon is described as Sam’s younger and more physically capable brother. Their father, the military commander Lord Randyll Tarly, cruelly forced Sam to choose between joining the Night’s Watch or death so that Dickon could be his heir instead. When we last saw Sam in season 5, he was leaving Castle Black with Gilly for Oldtown to study to become a maester.