fly on the wall:未被察觉的观察者
Shut up!I think there is a fly on the wall.
别说话!好象有人在偷听.
kill an elephant:费力气做（不必要的）事
He asked you to adjust the carburetor,not rebuild the motor.Don't kill an elephant.
他让你调调化油器,又没让你改装发动机.别白费力气.