Given scene/plot descriptions from movies or TV shows, and given a specific question about the scene, summarize the question into a title

Doc Brown has got a time machine, why is he in a hurry in the end of Back to the Future part 1? Why doesn't he just give a parenting lesson to Marty? A little more context: At the very end of part one, doc goes back to Marty to tell him about his son which is going to jail in the future and they need to stop that to happen. He is really in a hurry, but c'mon, you can wait a couple of days in the present then go back to the future at the very same moment he intended to go in first instance. He has plenty of time to achieve that!
Why is Doc Brown in a hurry in the end of Back to the Future part 1?

In "Edge of Tomorrow", Rita Vrataski (aka "Full Metal Bitch", "The Angel of Verdun") narrates how, after the Battle of Verdun, she lost large amounts of blood. After a blood transfusion she lost her power to "reset" her day, which only happens when she dies. How did she find out that she had lost her power? How did she find out for sure without killing herself?
How does Rita Vrataski find out that she lost her power to "reset" her day without killing herself?

In season 3 of Game of Thrones, Daenerys Targaryen buys Unsullied in exchange for one of her dragons. After turning over the dragon's leash, she is handed a decorated whip as a symbol of ownership. She then orders the Unsullied to "Slay the masters,... slay every man who holds a whip..." The Unsullied stand motionless as Kraznys mo Nakloz, the seller, screams back at them to kill Daenerys. She then commands the dragon to incinerate Kraznys, after which the Unsullied sack the city. Is the delay before the initial attack simply for cinematic effect? Or did Kraznys train the warriors with a fail-safe, to refuse any order to kill him, which required him to be dispatched first? If the whip gives command, how can Daenerys order them to kill other whip holders? Couldn't one of them countermand the order? Or are they motivated by revenge against their former masters?
How do the Unsullied prioritize their orders?