i will give you my best SWAG-
*they are trying to suppress a natural facial expression in response to something which has elicited in them a strong emotion that they are trying to suppress and/or hide from you. either you said something to them which they took as risible - OR you said something which offended them somehow, and they are trying to hide their anger from you either in the service of being civil or because they are trying to keep control of their emotions, IOW they're trying hard not to blow up at you. either way, it is not especially a good sign. if i were in that situation i would make myself scarce for a while.