I imagined David looking more like Percy Daggs III (Wallace from Veronica Mars).
I like Jennifer Lawrence as Megan, she would be able to switch between charm and coldness in a believable way.
I imagined the Prof with hunched-forward shoulders. Emaciated-looking, fragile and bird-like. I can't think of any mid 40s actors that have the look I'm thinking of, maybe Neil Patrick Harris when he's being Dr. Horrible.
Abraham I thought of Shepard Book from Firefly. Of course, Ron Glass is probably too old at this point to bulk up the way he would need to to fit this role.