The second level is to tell the story even if nobody asks. This is derived from the verse "and you shall tell your son on that day..." You should tell him, even if he doesn't' ask. Thus, the procedure of question and answer is preferable, but not absolutely necessary.

(This is a rare example of L'chatchila and B'dieved in a Torah Mitzva.)

The third level is: Even if a person is alone, he must speak about the going out of Mitzrayim. This is what Rabban Gamliel teaches when he says that one must "say" three particular statements as the bare minimum to fulfill the Mitzva. Thus, there are three possible levels on which to perform the Mitzva of "Sippur Yetzias Mitzrayim."