deadpan

他以一副面无表情的样子讲述了那个荒诞的故事。

He told the absurd story with a ___deadpan expression.

她用一脸严肃的表情讲了个冷笑话，这种面无表情的幽默反而让全场大笑。

Her ___deadpan humor, delivered with a completely straight face, made everyone burst into laughter.