This one’s.. fascinating. And probably spoilery. So we’ll just say it’s Littlefinger being threatened by a knife… held by Catelyn Stark! This is not, it should be said, a scene from the book.
Given the location, it sure looks like this is a meeting at Renly’s camp, with Littlefinger’s role as an envoy to Highgarden being changed to his being an envoy to Renly and so meeting Catelyn.
Previews, Petyr Baelish