The narrator comes directly into the poem twice. The first time, he's addressing us, the readers, when he's describing the party in the hall: "These let us wish away, / And turn, sole-thoughted, to one Lady there" (41-42). The narrator here is very conscious of himself as a narrator, and is in control of the poet-reader dynamic. He's the one guiding us through this castle and this story, and he can sweep away the great hall and zoom in on one of his characters because, hey, it's his poem.
The second time you see the narrator step into the poem is quite a bit later, when Madeline is entering her bedroom in stanza 22: "Now prepare, / Young Porphyro, for gazing on that bed" (196-197). Notice something different?
We, the readers, are no longer the audience of the speaker's remarks; Porphyro is. At some point in the poem, the characters in the poem have become the audience of their own creator. In speaking directly to Porphyro, you can also hear the speaker's own anticipation, his own excitement to look at Madeline—it's as if he's assumed the feelings and intentions of his own character.
This poem spends a lot of time thinking about the truth of the imagination and the possibility of becoming ensnared inside your own dreams. By the time the speaker speaks directly to Porphyro, it's as if he's forgotten that it's a poem and that he's the one writing it; it's almost like he exerted so much control over the poem's narration early on, just so he could ultimately lose that control.