have a straight face

To have a serious or emotionless expression on one's face, especially in a situation in which one would be expected to laugh or smile. I thought he was joking when he told me his ridiculous plan, but he had a straight face the whole time. It's so hard to have a straight face on camera when you have hilarious co-stars like I do. A: "So what exactly did you say about politics that got my aunts all riled up?" B: "I just made a few jokes, but I guess I had too much of a straight face at the time. Oops."