Potential Game of Thrones show spoilers and definite book spoilers below. You've been warned.

More and more over the past few seasons, Game of Thrones has been marching to a beat much different from that of George R.R. Martin's A Song of Ice and Fire book series. While many of the core plot points remain the same, characters have been cut out, events rearranged and, in some cases, entirely new plot points have been added.

Season six has proven to be no different. Already key characters are in locations far different from their book counterparts. In some cases the show is much further ahead in regard to the story arcs of certain characters. That being said, it's looking increasingly likely that the show is set to introduce two key book characters that have long been missing on the show: Lady Stoneheart and the mysterious Coldhands.

Lady Stoneheart first appears in the epilogue of A Storm of Swords, the third book in Martin's series, making her appearance long overdue.

Who is Lady Stoneheart? She's none other than a resurrected Catelyn Stark. After being murdered at the Red Wedding by having her throat slashed, she is discovered by the Brotherhood Without Banners, among them the red priest Thoros and resurrected Lord Beric Dondarrion. Several in the group ask Thoros to attempt to bring Lady Stark back to life. He refuses, saying that she has been dead for too long.

That's when Beric steps in. He gives the "kiss of life" to Catelyn, restoring her to life but in the process taking his own. After her resurrection, she assumes control of the Brotherhood, as they hunt down Lannisters, Freys and anybody else Catelyn deems responsible for the Red Wedding in the Riverlands. Because of the massive gash in her throat, she is barely able to talk, instead pointing and grunting most of her commands.

The reveal of her identity at the end of A Storm of Sword serves as one of the novel's key shocks, and she has a major role to play in Brienne of Tarth's story in the following book, A Feast for Crows. It's in that book where Lady Stoneheart captures Brienne and plans to execute her, believing Brienne to have forsaken her oath to Catelyn and to be a traitor working for the Lannisters.

In order for Brienne to prove her loyalty, Lady Stoneheart commands the knight to seek out and slay Jaime Lannister, who is nearby attempting to negotiate the surrender of Riverrun. While Brienne initially refuses to accept Catelyn's task, it appears she later gives in, in order to save both her life and that of her squire Podrick Payne. It's important to note that in the novels Brienne has yet to find Sansa, who is still with Littlefinger in the Vale.

For that reason many fans have assumed that the show has simply omitted Lady Stoneheart. After all, the show version of Brienne has reunited with Sansa and proven to be loyal to House Stark. She's currently in the North. Brienne's story line has taken a far different path from her novel counterpart, and the show is long past the point of the character's book introduction.

But that might be changing soon. As revealed in the latest episode of season six, Sansa is sending Brienne to the Riverlands to broker a deal between Catelyn's brother Brynden "The Blackfish" Tully, who has recently taken control of Riverrun. That puts Brienne in a position to encounter Lady Stoneheart (and possibly Jaime) were she to exist in the show. So is it possible her reveal will be coming this season?

Maybe. A Game of Thrones fan on Twitter reported seeing Catelyn Stark actress Michelle Farley at a local cafe in Belfast, where much of Game of Thrones season six was filmed. There were also season six casting calls for "outlaws" and a "son who is unable to live up to the demands of his savage father." That sounds like it could be referring to Lady Stoneheart's band of renegades as well as Merrett Frey, the son of Red Wedding mastermind Walder Frey.

In the books, Merrett Frey is executed via hanging by Lady Stoneheart for, well, being a Frey. The casting call in question also makes clear that the role is a "one-off opportunity" that will have the actor interacting with leading cast members.

A "one-off opportunity" is definitely casting call code for "this character is going to die," which Merrett most certainly does. As for leading cast members, it seems reasonable to assume that one might be Farley. As for others, it seems as if Thoros actor Paul Kaye will be reprising his role on the show, with fansite Watchers on the Wall even reporting that one of his scenes involves a hanging.

The last pieces of evidence to consider are the comments of Game of Thrones special effects guru Barrie Gower. He's made reference to a visual "gag" that has been pulled from season four and five, but that is apparently set to appear in season six.

Gower, of course, could be referring to just about anything, but it's worth noting that he is heavily involved in doing prosthetics for the show. Bringing to life a zombified Catelyn Stark, her head barely attached to her shoulders, would certainly require some fancy prosthetic work, which would seem to make Gower the show's go-to man were Lady Stoneheart set to appear in season six.

With five episodes left in season six, it all depends on how quickly Brienne arrives in the Riverlands. If she does indeed arrive this season, we could see Brienne's encounter with an undead Catelyn serving as the perfect way to end season six. It would without a doubt get fans talking.

The missing piece of the puzzle is Jaime. He's currently occupied with the High Sparrow in King's Landing. If the show resolves that conflict soon and Jaime is sent to deal with The Blackfish at Riverrun, Lady Stoneheart's arrival seems all but imminent, as it will likely foretell a showdown between Brienne and the Kingslayer.

Lady Stoneheart, however, is far from the only important character from the books that looked to have been cut from the show. The other is the character known as Coldhands. This seemingly friendly White Walker aids Bran, Meera and Jojen while riding atop a massive elk, helping the trio reach the Three-Eyed Raven.

Who, or what, exactly Coldhands is remains a mystery even to book readers. It's clear he is dead and somehow related to the White Walkers, yet he is intelligent enough to speak. Unlike the White Walkers and their wights, his eyes are black, not glowing blue. He also appears to be a former ranger of the Night's Watch, as his clothing is that of the brothers in black. The long-running fan theory is that Coldhands is none other than Benjen Stark, Jon's uncle and First Ranger of the Night's Watch. Benjen goes missing shortly after the series begins while ranging north of the Wall.

Seeing as how Bran, Meera and Jojen already reached the Three-Eyed Raven without the help of Coldhands in the show, fans thought the character might never make an appearance. That could be changing now, as Bran and Meera are fleeing for their lives with the White Walkers and the Night King in hot pursuit.

Where exactly Meera and Bran will flee to remains uncertain, but now seems the perfect time for Coldhands to make an appearance. The character is shown to be a deadly fighter in the books, as he cuts down wights and even traitorous brothers in black in order to keep Bran and his friends safe. He could certainly help protect Bran and Meera from harm.

One of the show's trailers does feature a black-clad rider in the North wielding a flaming flail against, what could be, a wight. Might this be Coldhands? Possibly, though if it is he looks considerably different from the book character. Aside from that, there's little real evidence to support the idea that Coldhands might appear in season six other than that it makes sense from a story perspective.

That being said, it is worth noting that Benjen Stark does appear in the show's first season. It's stated that the two men accompanying Benjen on his ranging are found dead. Benjen's horse is also discovered, though Benjen himself is never found. It seems almost wasteful for the show to introduce the character and then never explain his disappearance or possible death. For that reason it seems likely that Benjen still has some role to play in the events to come, either as Coldhands or otherwise.

Whether or not either one of these prominent book characters will make an appearance on the show is uncertain, but it's looking increasingly possible that season six might be the perfect time for each to make their television debut. With only five episodes left this season, fans will soon have their answer.