Adam has a very expressive face. You generally know what he's feeling because it shows. Not so much with Jamie. There's a couple of times he shows exhuberant joy (meh) but it's only recognisable because it's only slightly more than his usual neutral face.

As Adam explained it, Jamie's HEAD was an "emotional barometer" ... He's told the time that Jamie pranked him on a road trip, and he knew something was up, because Jamie's head went red (from him stifling his laughter). Like when someone's ears will flush red with embarrassment (or any strong emotion).