3 ideas
Desire is not abstract but anchored in daily visual exposure: we covet what we see every day. For an investigator, this means an offender's first act is usually close to home and familiar, so the earliest case, not the most recent, holds the richest evidence about who they are.
Instead of starting with categories or motives, ask of any phenomenon 'what is it in itself, what is its nature, what does he do?' and reason from the concrete behavior outward. Lecter's Marcus Aurelius method moves Starling from what Buffalo Bill is (a 'transsexual' profile) to what he does (sews a suit from women's skin), which reveals his real aim: transformation.
In a relationship with a manipulative expert, information flows only when both sides pay, so the questioner must reveal real vulnerability to get real answers. The exchange cuts both ways: the same disclosure that buys insight also gives the other party leverage over you, and it forces you to confront your own motives.