(Ibid. 18) "A prophet shall I raise up for them": We are hereby taught that as a reward for their fear (of listening to the voice of G-d) they merited that prophets arise for them.
(Ibid.) "and I shall place My words in his mouth": I shall do this, but I shall not speak to him face to face. ("And I shall place My words in his mouth":) From now on, realize that the holy spirit has been placed in the mouth of the prophets.
(Ibid.) "and he shall speak to them (directly)": Do not set up an interpreter for him.