Marty: "Okay, all right. Saturday's good. Saturday's good. I can spend a week in 1955. I can hang out. You can show me around."
Doc: "Marty, that is completely out of the question. You must not leave this house. You must not see anybody or talk to anybody. Anything you do can have serious repercussions on future events. Do you understand?"
Marty: "Yeah, sure, okay."
Doc: "Marty, have you interacted with anybody else today besides me?"
Marty: "I'm... Yeah, well, I might'se sort of bumped into my parents."
Doc: "Great Scott!"