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）