The pic was originally posted on Instagram by Gemma Whelan (who played Yara Greyjoy on the show) but has since made its rounds on Twitter.
Maybe Harington was just dusting mattifying powder on Williams’s forehead to prevent shine? After all, it looks quite warm, and the scene was filmed during the middle of the summer, in the middle of Spain. Issac Hempstead-Wright (Bran) isn't even wearing pants.
Or maybe Harington is just lining up a prospective career post–Game of Thrones, à la every single Bachelor contestant. It's always good to have options, and we'd be delighted to see the next "dewy dumpling skin" trend come from Jon Snow himself. Looks like we have a burgeoning YouTube beauty guru on our hands, don't we?