This thing is currently driving me insane. I spend hours writing dialogue and setting up a scene, and it plays completely differently in the sketchbook. I've resorted to over-simplifying settings and action, and breaking dialogue down to basic messages like "Hello, I want to bang your girlfriend" and "I am disgusted, and slightly intimidated by you, and I feel my ego is under attack." Seems to be working... okay, in that I'm not as frustrated. The thumb-nailing is more true to how how I want the scenes to play out, and it gives me more leisure to match a character's actions and expressions so I can play around with how they say things.
So yeah, it changes dramatically. But I'd totally take thumb-nailing over the script P: