Spoilers ahead for the most recent episode of Game of Thrones, as well as A Song of Ice and Fire.
Oh Bran, how we've missed you! We haven't seen this "summer child" since the end of season four, when he finally found his way to the Three-Eyed Raven, who turned out to be a man living in a weirwood tree. What has Bran been doing since then? What is his new power? And how will this be important for season six?
We've seen before that Bran has visions with a capitol V, but he hasn't had a lot of control over them — most of them were sent to him. This past year, he's been training with the Three-Eyed Raven and the Children of the Forest to unlock his mystical powers from the old gods: the power of the greenseers. "Greenseeing is an ability which several people have had over time," explained Isaac Hempstead Wright, who plays Bran. "It's one of the very mythical elements of the Westeros world, so there's going to be a lot of cool stuff for Bran this season."
How is greenseeing different than having visions, à la Jojen Reed? Jojen calls himself "only a boy who dreams," because his visions are "green dreams," prophetic dreams. The greenseeers, he insists, are "more than that." First, you'd have to be a warg — the ability possessed by Bran, as well as Orell and other wildlings, to mind-control birds and beasts. "Only one man in a thousand is born a [warg]," Bran is told. But Bran's not just any warg — he's also mastered taking over another human being, albeit a simple one. (No offense, Hodor!) That's not an ability other wargs are known to possess, and it's what takes Bran to an advanced level. The rarest and most difficult form of warging, however, is greenseeing, or the ability to slip into plants — specifically, weirwood trees. "Only one [warg] in a thousand can be a greenseer," Bran is told in the books. "The greatest of them could wear the skins of any beast that flies or swims or crawls, and could look through the eyes of the weirwoods as well."
Why would you want to do this? Because the trees remember things men have forgotten. To commune with the weirwoods, Bran goes through the roots and follows them up through the earth, and the trees reveal the entirety of their lives and what they've witnessed, especially if they had a "face" carved into them. "Time is different for a tree than a man," Bran is told in the books. "For men, time is a river. We are trapped in its flow, hurtling from past to present, always in the same direction. The lives of trees are different. They root and grow and die in one place, and that river does not move them. The oak is the acorn, the acorn is the oak." Through a tree, Bran is able to gaze into the past, "be it yesterday or last year or a thousand ages past." He might even be able to see the future. "The idea is that you look through the eyes of the very same weirwood tree throughout its entire timeline," Hempstead Wright said. "It's a neat idea, to harness the power of the weirwood trees to look through time."
Watching Winterfell — for example, seeing his father, aunt, and uncle play as children — is just the beginning. With practice, Bran should be able to direct his visions to trees and godswoods beyond the ones he knows, to see beyond the people he knows, to recognize and interpret even beyond his immediate knowledge. "A man must know how to look before he can hope to see," he's told. This might lead Bran to a vision of the Tower of Joy, a much-anticipated flashback, as well as an encounter with the White Walkers, as glimpsed through the trailers. Hempstead Wright couldn't reveal much about those vision quests, only to say that the Night's King stunt performer helped make the scene "absolutely terrifying."
The difference between warging into a plant versus an animal is that Bran cannot affect the tree (or what he witnesses). He can't interact with the human beings he watches, at least not yet. Should he try to speak, his words might sound like the wind rustling the leaves. This may not necessarily stop him from trying, and maybe a few people will hear him speak their names. Or maybe they think they've imagined it, a voice on the wind. They may not recognize what's happening.
"What's nice about the magic in Game of Thrones is that it's treated in the same way magic is treated in our world," Hempstead Wright said. "They're skeptical about it. Dragons? There are no dragons anymore. The White Walkers don't exist anymore. This red witch is crazy. So when you do see magic that is real, especially serious magic like Melisandre or the Three-Eyed Raven, it makes it special."