a fine line

US /ə faɪn laɪn/
UK /ə faɪn laɪn/
"a fine line" picture
1.

一线之隔, 微妙的界限

a subtle distinction or difference between two things that are very similar but can have very different consequences

:
There's a fine line between confidence and arrogance.
自信和傲慢之间有一线之隔
It's a fine line between being helpful and being intrusive.
乐于助人和多管闲事之间有一线之隔