a fine line

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

тонкая грань, неуловимое различие

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.
Есть тонкая грань между уверенностью и высокомерием.
It's a fine line between being helpful and being intrusive.
Это тонкая грань между тем, чтобы быть полезным и быть навязчивым.