In this case, it is wise to be a fly on the wall.

在这种情况下，还是做个置身事外的观察者比较明智。

A fly on the wall alludes to the position of being able to freely observe a situation without being oneself noticed.

“墙上的苍蝇”指在不被人注意的情况下随意观察局势的一种状态。

She was always distracted by something: a fly on the wall, an invisible squirrel running up a tree, a dirty old sock under the couch.

随便什么东西都能让她走神：墙上的一只苍蝇、一只飞速窜上树梢无迹可寻的松鼠，或者是沙发底下一只又脏又旧的袜子。