One of the women disrobed by The Red Viper on tonight's episode is Josephine Gillan, who was shown completely nude from the front and then from behind. She had previously done nude scenes in the first 2 episodes of Season 2. There was another actress being disrobed, but her identity has not been confirmed as of right now.
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From The Daily Beast:
"here’s a great scene in the premiere, and I won’t give it away, but it involves you casually disrobing some women. Where do they find all these nude extras for Game of Thrones? ...Just a flick of my finger and dresses fall to the floor! Well…they’re actors that the casting directors find that are brave enough, and very well taken care of. It’s set up to be very scary and intimidating, but it turns out to just be loads of fun. That stuff was particularly fun."