The tight reg who calls your river bet and then immediately looks away is not happy about it. He called because he had to, not because he's confident. He's not trapping - he's done.

Why they do it

Live body language after a river call tells you a lot about the next hand, not the current one. A tight reg who calls reluctantly and looks away has just told you he's not confident in his hand. He wasn't trapping on the river - he was pot-committed and unhappy about it.

The adjustment

Note the reaction. Next hand against this player he's still processing the last one. He might play tighter or he might spew trying to get it back. Either way, adjust accordingly.

What it's worth

Reading post-hand body language correctly gives you a head start on the next hand against this player. That context is free information most players ignore completely.