OP is living under a rock. Have you seen Focus? that chemistry was real....]
Body language...
Yeah the tells are there. Eye contact - Margot leaning towards him; legs crossed towards him.
Also, Margot's reaction at ~2:51 when the interviewer asks about keeping the private life private and staying out of the tabloids (gets fidgety, adjusts to not lean as much, both of their mouths struggle to keep the smile in the corners for a little bit, Margot's blink rate goes way up). You can even see the muscle on the side of Will's neck tense up and stay tensed when responding whereas that doesn't happen before in the interview.
Literally, cover up everything but their eyes or their mouths with your fingers and run 2:49 through ~3mins when they recover and tell me the question / topic of privacy and private life isn't uncomfortable for them. They recover there's a good 10 seconds of fight or flight going on.