Tā nà yìběnzhèngjīng de biǎoqíng ràng rén rěnjùn-bùjīn.
His super-serious expression made people unable to keep a straight face.
Tips
mistakes
Common pitfall: 俊 here doesn't mean 'handsome' - it means 'smile, laughter' (archaic sense). 忍俊 = 'hold back laughter'; 不禁 = 'cannot restrain'. This idiom is specifically about trying and failing to suppress a smile.
register
Literary but not stuffy; fits in modern writing about humorous scenes, animals, children. Slightly more polished than 笑了起来.
