At our local theaters, we go online and select our seats from a map of the available seats. From your computer or smart phone, the payment is made and you go to a kiosk and have the card read that was used for the payment. You can also print out a sheet with a bar code that can be read by a scanner. The kiosk will print out your tickets and, from there, you go to your seats. These seats are like recliner chairs, which will tilt you back and lift your feet up in padded luxury.
The last time my wife and I went to the movie, we came up to our reserved seat and a couple was in our seats. I asked if they made a mistake and they told me they came in late to their movie and wanted to watch the part they missed. Thankfully, they found an empty pair of seats and we did not have any trouble. However, if it was a drug-addled member of the hood, there could have been a different outcome. I have heard of situations where people tried to see a movie twice and the police were summoned to usher the offender to a police car.
There is no reason a person should be in an other’s reserved seat. If they are in your seat, you show them your ticket with its seat number and the time of the showing. If there is any dispute after that, the management or cops should handle the situation.
Would be nice if you could know what sort of people will be sitting in the surrounding seats. ie, "John Smith, a loudmouth jerk from the neighborhood will be occupying the seat to your immediate right"