Arguably the two most famous people on Game Of Thrones used to hook up back when the show first started.
Then he started seeing someone else on the show who is not as famous and the hookups fizzled out.
Apparently though he is now cheating on that someone else with the first co-star again.
Emilia Clarke (Daenerys Targaryen)
Kit Harington (Jon Snow)
Rose Leslie (Ygritte)