This post contains spoilers for the Season 6 premiere of Game of Thrones.
The big twist at the end of "The Red Woman" was not that the episode's titular character was bringing Jon Snow back to life, as many hoped, but that she is actually much older than she appears.
When she removed her choker (which glowed red as she did so), she appeared elderly and frail, suggesting the necklace has the power to mask her true form.
But some fans have pointed out that in this scene in Season 4, Melisandre was taking a bath without her choker and still looked young and flawless.
Selyse has this strange expression the entire time. Like she cannot look directly at Melisandre yet also cannot look away. At first you think it is that of a religious "uptight" person seeing a naked and very sexual woman. But if you rewatch the scene, and "pretend" to yourself that Selyse is seeing Melisandre as she actually appears, it actually fits better. WE still see the seductress, Selyse sees the hag. She is horrified yet fascinated and not at all shocked.
The theory makes a lot of sense considering Melisandre is discussing the tricks and lies she uses to convert people in this scene, emphasising that Selyse doesn't need these because "you are strong enough to look into the Lord's light and see his truth for yourself. However harsh it is."
Still, this doesn't really explain why we as viewers see young Melisandre in this scene.
Not convinced? OK, OK, this theory might suit you better:
Reddit user I_Hate_Nerds comes to a different conclusion from the idea that Selyse is a "true believer" – that she (and therefore the viewer) is actually seeing Melisandre's ideal young and beautiful form even without the necklace due to the strength of her faith. If this is the case, it still ties in nicely with the idea that the big reveal happens at the moment when Melisandre's faith is lost.