I've watched a fair bit of film that deals with time travel.  Although some truly stand strong against the test of realism, for the most part, they just do not physically work.
Time travel, believe it or not, is something that one day will be possible.  There will be actual rules to it.  Right now, the most likely belief in the scientific community is that it will not be possible to travel back to the time of the dinosaurs, or back to meet Shakespeare, or even to the year 2007.  No, time travel will only be possible back to the date that its conception was realized.
Now, I realize that SciFi is all about things that have not yet happened, or even never can happen.  So it's allowed to present the situation where time travel back to any date is possible.  Fine.
But the rules of time travel are more intricate than any other world.  Time travel may not be possible for reasons other than physics.  What I said above, that one day it will occur, well, I mean that physically.  Physics allows time travel to happen, assuming we can find a way.  However, there may be reasons other than physics that stop us.  What does that mean?  Well, take the grandfather paradox.
For those of you that are unfamiliar with the grandfather paradox, read this paragraph.  If you know of what I speak, skip down to the next.  The grandfather paradox is a paradox created through time travel where one goes back in time, and kills their grandfather before his own father has been concieved.  Naturally, this means our time traveller was never born, and never able to go back in time and commit this crime.  But, if he never went back, then his grandfather DID concieve his father, and thus, our trime traveller was born, and able to go back to kill his grandfather.  The circle continues infinitely.
What does the grandfather paradox mean?  Personally, I believe that (physics aside) time travel cannot happen.  Such a paradox would throw the universe out of alignment.  It would simply cease to be.
I can't even BEGIN to explain the paradoxes that Journeyman brings up.  Well, I'll explain just one.  He reads a paper that says 57 people killed in earthquake in 1989.  Our hero goes back in time.  Tries to save people, fails.  Comes back to present day.  57 people were killed ine Earthquake in 1989.  He feels he failed.  Theoretically, he shouldn't know that.  Say originally it was 59.  So he saved 2 people, but he SHOULD NEVER KNOW.  Since those people never died now, what he'd remember is reading a paper that presents that truth!
I hope that makes sense.
To all aspiring story-telllers... if you plan to write anything to do with time travel, either stop now, or realize that you are going to have to spend years and years verifying physical facts about how time travel could theoretically work, because problems like the one(s) in Journeyman blow holes in the entire plot.
(For the record, I believe the third Harry Potter book, and movie, are VERY good examples of how time travel should work)