all of a sudden和immediately有什么区别?

all of a sudden,
If something happens all of a sudden,it happens quickly and unexpectedly.
All of a sudden she didn't look sleepy any more.突然的意思（表示事出意外）
immediately,
If something happens immediately,it happens without any delay.
He immediately flung himself to the floor...
Ingrid answered Peter's letter immediately.立即,立刻,马上的意思（表示动作快）