Usually, a person is clowning around if he is being funny when he should be serious.

VOA: special.2009.01.11

I'm being serious now, goddamit. How can you miss seeing that?

我现在很认真，该死，你们怎么能没看到它呢？

耶鲁公开课 - 1945年后的美国小说课程节选

He normally, you know, I can't really get used to him being serious, you know.

一般情况下，我真的不习惯他这么严肃。

英国电影的魅力 - SpeakingMax英语口语达人

But his health took a turn for the worse in early 2000 and he needed to get serious about being upgraded on the list.

VOA: standard.2010.02.26