With the price of a theatre ticket costing you an arm and a leg these days, it is awfully unfortunate if we can't enjoy the movie we have chosen (and paid) to see due to someone in front of you playing musical chairs during the course of the movie.
Arrive well before the cinema lights go down and prepare yourself prior to the movie starting. Buy snacks, turn off your cell phone (or mute the ringer) and visit the bathroom in advance, giving you enough time to find your seat while the cinema is still brightly lit. If the lights have been dimmed before you have found a seat, quickly scan the screening room as best as you can and find yourself a seat pronto. Most movie theatres still employ ushers so seek one out and get their help in finding you a seat.
If you absolutely must get up during the movie, quietly and quickly is the way to leave and return to your seat.