Dean-Charles Chapman is currently known on Game of Thrones for playing King Tommen Baratheon, first of his name and however it goes, who took the throne in Season 4 after his brother Joffrey's death. For reference, that's Tommen on the left and Martyn on the right: 
 
 Image: HBO 
 
 It's an easy thing to miss because poor Martyn Lannister only appeared for two episodes in the third season before getting the axe. For a refresher, Martyn (along with his brother Willem) was a squire in the Lannister army who was taken prisoner by Edmure Tully at the Battle of Stone Mill (events that took place off-camera). Also, sadly, Martyn and his brother were killed off by Lord Rickard Karstark in revenge for Jaime Lannister killing his own son. Well, okay, not really, not in the world of Game of Thrones. Hello Giggles first caught the similarities, noticing that the the king who's "only half-Lannister" looks exactly like his pure-blooded Lannister cousin (and also, IMDb says so).
– The internet loves obsessing over Game of Thrones, so Hello Giggles wants to know why more people aren't talking about the fact that "Tommen is also his own cousin." Dean-Charles Chapman currently plays King Tommen Baratheon on the show, a role he took over from a younger actor in 2014. But he also played Tommen's cousin, Martyn Lannister, in a few episodes of the show back in the third season, Mashable explains. Hello Giggles thinks the recasting makes complete sense. “Given the Lannister family’s penchant for incest, having several people in that lineage look the same (like exactly the same) would not be a surprise.” Poor Martyn was killed off after being taken hostage by Robb Stark, and Bustle thinks Chapman better hope for a third role as Tommen is "probably doomed just like his look-alike cousin."