First of all, I am totally impressed you did this with markers -- I feel like a kindergartener, I'm so inept with them -- you get so much color subtlety out of something that lends itself to discrete blocks of color. Seriously, wow. :D
I LOVE the costume design, too -- Snape's long coat with wide lapels and vest call back to his signature black robes, but they fit perfectly into the detective genre. And Harry! You know he'll just wear jeans no matter what. <3 :) Also: GLOVES OMG. They are just gorgeous :D I love that seam along the outside of Harry's hand -- it's such a nice touch. I'll have to remember that the next time I draw gloves: add a seam so it doesn't look like I just colored in the hands. ;)
Finally, oh, the looks on their faces! They look like they're reeling from OMG first kiss feelings and are desperately trying to go back to the hard-edge detective poker face before anybody notices -- Snape, of course, being much more successful. Harry still looks totally seductive. :D