Who is the narrator, can she or he read minds, and, more importantly, can we trust her or him?
Third Person Omnipotent
The narrator of The Romance of Tristan is way omnipotent: at several points in the story, he tells us the fate that awaits his characters long before it comes to pass. He particularly delights in revealing the horrible deaths in store for his villains, like the dwarf Frocin or Mark's barons. The obvious pleasure he takes in punishing those who have it in for Tristan and Yseut reveals another characteristic of our narrator: he's emotionally involved in the story. Like, totally involved.
He shows us how involved he is when he criticizes the actions of his villains, like when he asks who would think of such a "low trick" as Frocin's. We also see it when he laments the missteps of his heroes, like when he interjects "God, what folly! He was too rash" about Tristan's spectacular leap to Yseut's bed.
The narrator's emotional commentary allows him to direct our sympathies and interpret the story for us as he tells it; it's also a narrative style typical of oral poetry. (That would be poetry that is spoken or recited rather than written down.) Whenever he says "Listen!" or "Hear how . . .," the narrator creates the feeling of oral performance. In fact, this scene may not be entirely fictional: it's possible that The Romance of Tristan was composed and delivered orally before it was written down.