Unquote #27 answer

The quote

Oh, come on. This is Sunnydale. How bad an evil can there be here?

The puzzle as it was set

FI, QFUN FP. BIXZ XZ ZAPPCKTVN. IFG DTK TP NOXV QTP BINYN DN INYN?

The key

Each code letter stood for one real letter (code letter, then the real letter it hid):

T → AD → BQ → CK → DN → EI → HX → IV → LU → MP → NF → OY → RZ → SB → TA → UO → VG → WC → Y

The most frequent code letter was N (8 times), which stood for E. Short words, apostrophes and repeated letters are the usual way in: a one-letter word is almost always A or I, and THE is the commonest three-letter word in English.