Deliberately impassive or expressionless.
Impassive.
Having such a face or look.
To express (oneself) in an impassive or expressionless manner.
In a deadpan manner.
A style of comedic delivery in which something humorous is said or done while not exhibiting a change in emotion or facial expression.
Not fussable; (especially) level-headed and difficult to frustrate or provoke.