Symbol Analysis
"The Eve of St. Agnes" swings wildly between a series of stark contrasts: hot and cold, music and silence, awake and asleep, old and young, Christian and fairy—you can go on and on. You can actually analyze the poem's elements simply by tracking their position between these contrasts as they progress through the poem. For instance, Porphyro is consistently written as hot and fiery, but when Madeline awakens in her bed he's made out to be "chill and drear" ( 311), signaling to you that something funky is going on.
The poem certainly feels more dynamic for these contrasts—as we've mentioned before, not a whole lot happens in "The Eve of St. Agnes"and it reads slowly (there's on the pacing in "Form and Meter"), so the shifts from hot to cold, dead to awake, etc. create movement that doesn't otherwise exist. On another note, it simply heightens the drama: it's not cold, it's utterly freezing; Angela isn't just a little lonely once Madeline and Porphyro leave, she's dead.
Another effect is that, having fielded all of those contrasts for many a stanza, you notice the ways in which those contrasts start to break down as you approach the poem's climax. Once Madeline enters her "wakeful swoon" (236), things get pretty murky: Is she awake or asleep? When she opens her eyes in stanza 34 and looks at Porphyro, is she seeing a dream or a real person? Keats has been setting up binaries for so long that, when the real imagination par-tay gets started in Madeline's bedroom, you're very sensitive to the ways in which those binaries start blending into each other, preparing you for that all-important moment where Porphyro "melts" into the dream of Madeline (320).