Daenerys’ paramour was played by British actor Ed Skrein in Season 3. However, he was replaced by Dutch star Michiel Huisman for Seasons 4 through 6. It's a little unclear why Skrein left (it was suggested he left the show to do the film Transporter, but Skrein maintains it was "a lot more political than that").
2. The Night King
For Seasons 4 and 5, the Night King was portrayed by actor Richard Brake. Since Season 6, however, he’s been played by Slovak actor/stuntman Vladimír Furdík — whom we've also seen in "human" form when the Children of the Forest turned him.
3. Beric Dondarrion
Aussie actor David Michael Scott actually portrayed Beric in a blink-and-you’ll-miss-it scene back in Season 1. He was later replaced by Richard Dormer who has portrayed the role since Season 3.
4. Dickon Tarly
Freddie Stroma played Dickon Tarly in Season 6, but was cast in ABC's Time After Time and had to drop out of Thrones, so Tom Hopper and his abs stepped into the role. By the way, you might recognize Stroma as Cormac McLaggen from Harry Potter and the Half-Blood Prince.
5. The Three-Eyed Raven
Veteran British actor Struan Rodger played the Three-Eyed Raven very briefly at the the end of Season 4, but when the show returned for Season 5, Academy Award-nominee Max Von Sydow took over the role.