'Game of Thrones' Season 6 Spoilers: Show Alum's Presence in Belfast Sparks Rumours of Lady Stoneheart Storyline

Rumours are rife that the Lady Stoneheart storyline will be explored in the upcoming season of HBO's "Game of Thrones."

Lady Stoneheart is the re-resurrected Catelyn Stark, who was killed along with her son Robb in the infamous Red Wedding. She has come back to seek revenge from the Freys, Boltons, or Lannisters.

Season 5 of the fantasy series, based on George RR Martin's "A Song of Ice and Fire" series of books, ended with Jon Snow apparently dying after being stabbed by his Night's Watch comrades. There is still a lot of mystery surrounding Jon's death, with many fans predicting he will be brought back to life with magic.

Actor Kit Harington's presence in Belfast, which is one of the filming locations, has only aided these rumours.

Interestingly, it's the presence of another actor and a new casting call that have sparked rumours of Lady Stoneheart's return.

Watchers on the Wall reported that actor David Bradley, who plays Walder Frey, was photographed by a fan arriving in Belfast airport, probably to film scenes for the sixth season of "Game of Thrones." According to the fan site, the inclusion of Walder Frey, and the casting call "for a son who is unable to live up to the demands of his savage father" could make Lady Stoneheart's return possible.

The son mentioned in the casting call could be Merrett Frey, who is one of Lady Stoneheart's victims.

Another potential clue was given by prosthetics supervisor Barrie Gower, who said in an interview that a special effects sequence that was initially planned for season 4 was pushed to season 5, and then it was pushed for the next season yet again.

"Benioff and Weiss, when they write the outline each year, for last two years, we got to this one point and we read this one gag and thought 'Oh my God, how the hell are we going to do that? But this is going to be incredible!' And then it got pulled and we didn't do it in Season 4. And then, lo and behold, in Season 5 there it was again. And it's pulled again! But now, in Season 6, it looks like we're going to do it. Fingers crossed they won't chicken out," Gower said, according to ComingSoon.

This quote has sparked speculation that the prosthetics could be for Lady Stoneheart.