Reigen stared at him, his expression stern in the way Serizawa had learned it was when he didn’t have to keep up his customer service pleasantries. He stepped close to him and rested his forehead against his shoulder, hiding his expression. His quiet words, however, Serizawa could hear loud and clear.
Shipping goggles aside, I believe Petyr’s look when he’s seeing Sansa after being choked by Jon, is him wondering how strong of a bond/loyalty do they have for each other. He seems to only see them fight in the throne room so he believes that they are distant, have differences and cannot compromise. However after the crypt scene, he’s confused in whether Sansa is truly on his side or if he’s going to be able to pin them against each other therefore plotting another sibling rivalry which could potentially be Sansa and Arya.