play it by ear 随机应变，见机行事，根据情况需要行动

【例句】
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。

Let's play it by ear when we get there.
到了那儿，我们要见机行事。