In Back to the Future, Doc invented a time machine in 1985 and he was really excited and a little surprised that it worked. But later in the movie, Marty goes back and tells young Doc about it.
Why was Doc so surprised about it, when he knew he would be inventing it in 1985 since 1955?
But a more serious question, if he knew it since 1955, wouldn't this knowledge change the date of the first time travel (make it sooner, because he had lots of information from the future, or even make it later, because he "knew" he would invent it so he could stop trying)?
Or why didn't he change the place of the meeting with Marty, to prevent the attack from the Libyans (any events in the past could change that moment, when Marty escaped only because the gun has jammed). Is it because he did not want to ruin the time-space-continuum?
I know this is not one specific question, rather a few confused questions, but I hope you understand what to me is vague about this movie.