How to dress, according to the book: William Peter Blatty wrote, "His eyes locked, stunned, on the thing that was Regan, on the creature that was lying on its back in the bed, head propped against a pillow while eyes bulging wide in their hollow sockets shone with mad cunning and burning intelligence, with interest and with spite as they fixed upon his, as they watched him intently, seething in a face shaped into a skeletal, hideous mask of mind-bending malevolence. Karras shifted his gaze to the tangled, thickly-matted hair; to the wasted arms and legs; the distended stomach jutting up so grotesquely; then back to the eyes."
How to act: I mean, Regan is literally possessed by a nasty demon. Don't go too crazy, but just know the character speaks in many tongues, uses a low, guttural voice, and liberally slings out insults and swear words.