What you need: A tank top, an awful Hawaiian shirt (or a leather jacket), aviators, pants, a bunch of empty pill bottles, an iPhone or something, speakers
How to pull it off: Put on the pants, aviators, and the tank top (topped with either an awful Hawaiian shirt or a leather jacket). Sleep in them until you need to wear the costume; it'll add to the illusion. Next, record a bunch of noir nonsense onto your iPhone's recording app--you know, stuff like "The candy reminded me of a childhood spent in dentists' offices, with drills piercing my teeth like daggers and short-skirted hygienists telling me I'd been a bad, bad boy." Then, play these out of a speaker as you walk around the party, popping M&Ms (or M&M Minis. Do they still make those?) into your mouth from the pill bottles.