To Amy, ringing Desi was a last resort when her savings were stolen. She called him because she knew he was someone who would help unconditionally and given how skittish she was initially in his lake house, it doesn't seem that it was always her plan to kill him. That point seems to be when Nick gives the interview on TV; she sees he does understand her and realises she can go back to marriage and be free. Cue one of the most gruesome scenes in Fincher's entire filmography. But what of Desi. Was he the creep he will forever be known as or just another innocent roped in by Amy? We know from Scoot McNairy's Tommy O'Hara that previous claims she's made against people are entirely fabricated, so it's not unreasonable to think the same is true of Desi (who allegedly tried to kill himself in Amy's bed). And yet when the duo are in the lake house it's almost a game of who is the creepier of the two. However, really look at what we learn about him. Desi is played a bit off-kilter, yet his actions are all pretty explainable. When confronted about his past with Amy as a student, he attempts to immediately get rid of Nick, but that could be out of embarrassment of the truth or anger at the lies. And when Desi has Amy back at his he may be drinking excessively (to build confidence) and immediately be rather touchy-feely, suggesting an intimate reunion is already on his mind, but given how he's kept his distance until she called, it's likely he wouldn't have made a move if she hadn't first. With the letters the two shared keeping him on the hook, really he's just another emotionally manipulated man. The ambiguity isn't helped that Desi is played by Neil Patrick Harris, an actor best known for despicable womaniser Barney Stinson. It's a move of smart casting that, despite there being no tangible link between Desi and Barney, we assume the characters are coming from the same psychological place when in fact that doesn't seem to be the case.
