play by ear 见机行事，随机应变;（不看乐谱而）凭记忆演奏（乐曲）

【例句】
We don't know what to do now, so we must play by ear.
我们不知怎么做，所以必须见机行事。

In dealing with that unpredictable man, you simply cannot follow a prearranged plan, you have to play it by ear.
和那个捉摸不定的人打交道，你简直无法按预定计划行事，只好随机应变。