Back to the Future
One of the movies I know every line of. That’d be Back to the Future, not — say — Top Gun. (All I know about Top Gun is that there’s some airplane flying and some guy makes a noise with his jaw.)
Actually, I’m hard-pressed to name another movie I “know every line” of.
I contemplate a few things, though.
What, exactly, is the backstory of Doc Brown and Marty McFly’s friendship? What opportunity does Marty McFly have, at what point growing up, to meet and start hanging out with a slightly mad scientist?
They filmed a third of the movie with Eric Stoltz, before they could wriggle out some contractual problems with Michael J Fox. What would the implications of this actor be on the movie? 
And finally — the subtle change to McFly being afraid of being called a “Chicken” in the sequels to the fear of rejection (comes out of George McFly) in this one? (In a way, doesn’t that message contradict the message of George McFly’s standing up to the bully Biff through punching him out — I note the reality that Biff is presumably physically stronger than George — then again, Marty has less reason to go with the game of Chicken.)
From the Wikki entry on said movie:
Universal Studios executive Sid Sheinberg sent Robert Zemeckis a letter intending to retitle the film Space Man from Pluto, because he didn’t think that anyone would care to see a movie about time travel. Zemeckis contacted Steven Spielberg for help, who wrote Sheinberg a letter. In it Spielberg thanked Sheinberg for the humorous letter and said that they all really had a good laugh about it, knowing that Sheinberg would be too proud to admit that he was serious. The issue was then dropped and never brought up again.