As Game of Thrones season openers go, Arya Stark's (Maisie Williams) mass poisoning of the loathsome Freys – just seconds after she'd reminded them of how they'd slaughtered pregnant Talisa and Catelyn Stark, "a mother of five", at the Red Wedding – was a brilliantly effective one.
Throughout the scene, Arya was disguised as Walder Frey (David Bradley), whose throat she'd slit at the end of the previous season. As per the previous "Faceless" transitions we've seen on the show, she looked exactly like the head of the Frey House, grey hair for grey hair, wrinkle for wrinkle (no offence, Walder) – before she reached up and smoothly peeled back his borrowed face, revealing her own visage underneath.
The question of exactly how Arya's face changing skills work, however, remains something of a mystery.
The younger Stark daughter mastered the technique while she was training with the House of Black and White in Braavos, learning how to be one of the mysterious assassins known as the "Faceless Men", and serve the Many-Faced God of death.
The Faceless Men, as depicted in the show, have access to a great hall of faces, all of them belonging to the dead. They are able to wear these faces, temporarily changing their appearance to look like the dead man, woman or child in question.
Arya eventually decided that remaining with The House was not for her; doing so would have required giving up her own identity and carrying out assassinations at the behest of strangers, instead of embracing her birthright as Arya Stark of Winterfell and working her way through her own, deeply personal kill list. She also incurred the wrath of The Waif, one of the acolytes of the religion, after she refused to carry out a commission.
But judging by the events of last season, and the most recent episode, it seems that while Arya might have rejected the ideology of the Faceless Men, she has retained their techniques, and is now employing them for her own ends, arguably making her one of the most dangerous people in Westeros.
So is Arya literally wearing somebody's skin?
While the show hasn't offered much in the way of explanation, we know that there is a physical element to the Faceless transitions. We see Arya peeling back Walder's face after she borrows it, for example, while in previous seasons, we learned that the cured faces hanging in the House of Black and White were removed from those who died there. We also saw Arya kill The Waif in season six, and then hang her sliced-off face in the hall.
That said, these aren't crude Leatherface-style masks: a magical element is clearly also involved.
In George RR Martin's source material novels, the scene in which Arya first dons a face that is not her own describes how her own face is initially cut, so it bleeds, before a new face is softened by the blood and pulled on to her head, magically fusing with her own.
While wearing the new face, Arya is temporarily able to experience the memories and nightmares of the young girl it once belonged to, again indicating that the process is a supernatural one as well as a physical one.
Whatever is going on, however, it's clearly very different from the glamour that Red Priestess Melisandre (Carice Van Houten) uses to make herself appear young, which seems to be linked to the necklace she wears, and does not involve the use of anyone else's features and/or corpse.
Does Arya have to physically be in possession of a face in order to wear it?
In all honesty, fans seem a little split over this point, although almost all agree that a person must be dead before a Faceless assassin (or Arya) is able to steal their features.
While some have argued that physically possessing the skinned face is a prerequisite to using it, this raises interesting question about just whose face Arya was wearing when she murdered Walder Frey last season, disguised as a serving girl. Had she killed an innocent young woman to borrow her appearance (arguably a very out-of-character move for Arya) – or did she simply find a corpse on her way there, or steal a selection of faces from The House before she left?
Others have suggested that, now that Arya has learnt how to be a Faceless assassin, she has automatic "access" to The Hall and is able to don any face from there she chooses, regardless of where she is geographically.
Previous episodes have also hinted that the faces can be fluidly swapped at will, and used by more than one person at a time.
Remember the season six scene in which Arya saw her Faceless Man mentor Jaqen H'ghar (Tom Wlaschiha) die, before The Waif removed her own face to reveal Jaqen's underneath? (Not to mention the rather Star Wars-esque hallucinatory sequence that followed, in which Arya removed a succession of faces from the dead man she had believed to be Jaqen, before finally seeing her own?)
In other words, our real answer to the question of how it all works is a resigned "(Many-Faced) God knows".
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