Emilia Clarke and Kit Harrington. Picture: Emilia Clarke / Instagram

Emilia Clarke and Kit Harrington say their 'Game of Thrones' sex scene was "weird" and their characters will be shocked to find out they are related.

The brunette beauty, who portrays Daenerys Targaryen, and her 30-year-old co-star, who plays Jon Snow in the HBO fantasy series, admitted that gagging is probably the appropriate response to the love scene because the pair do not know that Daenerys is actually Jon's aunt.

Speaking in a special behind-the-scenes commentary for HBO, Emilia said: "As actor's, it's just weird.

"The reality of what they are to each other, I don't know how that's going to... I think [gagging] might be the reaction."

Kit added: "I think they both know it's wrong. I think they both know it's going to cause problems. But it's that thing, when you suddenly feel that deeply about someone and you go through these events together, it's like a runaway train."

However, Kit believes it was "inevitable" that their characters would end up in bed together.

He said: "I think it's an inevitability from about halfway through the season that Jon and Daenerys will fall into bed together. You can't stop it from happening."

Emilia and Kit have both played major parts in the franchise since it first aired in 2009, but in the last eight years the pair had yet to shoot a scene together until recently.

Emilia explained: "Both of us were going, 'Ahhh what are you doing on my set!?'This is weird!"

Kit added: "We were both kind of freaking out.

"Usually you start working on a movie and meet [a co-star] for the first time and you develop chemistry on screen over time. Here, you know somebody for seven years and you've watched their character on screen all that time. So it's a unique experience as an actor to come together and know the world is watching"

However, the London-born heartthrob knew the pair's characters would finally meet in the current season of the show. 

He explained: "I predicted, like everyone, they would meet this season, but I don't think I predicted it quite as quickly. But then again, you've got to take yourself out of the mindset of the viewer. As far as Jon knows, he's just meeting this queen he's heard of and trying to negotiate with her -- he's not meeting Daenerys, who the audience has been watching for so many years. That helps with the surprise of it. He walks into the room and doesn't expect to see such a beautiful young woman of similar age to him. Any young man's reaction is going to be, 'Okay...' but he puts that aside because he has to."