I took a Ryanair flight two weeks ago and many people were sitting in the wrong seats. One woman had a tantrum about it while a man was more passive aggressive about it. What would you personally do if you were travelling alone? Would you find somewhere else to sit or would you ask the person to move? I would probably sit somewhere else but if I'm asked to move from that seat I would politely ask the person sitting at my assigned seat to move.
Choosing my seat (window) always costs me a lot of efforts, such as checking in when the possibility of doing it online just opens up, sometimes in the middle of the night. Or find a way to contact the airline when seat selection is not possible, for one reason or another.
Therefore, I am polite but strictly intractable if someone occupies my seat. A smile and a simple "hello, I think this is my seat" is all it takes, most of the time.
It happens quite often, especially when I board last or close to the last. So much so that I sometimes avoid boarding too late for this reason.... It's not always very pleasant to get a family moving, for example.
What's wrong with you people?! Why on earth would you approach the person sitting in your assigned seat directly?
It happened to me once on a SQ flight. I saw from a few rows down that someone was sitting in my seat..
I walked up to a flight attendant and told him someone was in my seat, showing my boarding pass..
So of course the attendant went up to him, asked for his boarding pass and escorted him to his assigned seat.
With this method, the offending passenger 1) has to comply as its an instruction from a crew member 2) you don't have to deal with aggression/passive-aggression/disgruntled faces etc
The problem with your method is that the FA will probably think (and tell to others): "Careful, this guy is a dick, bothering us for nothing". And then you will finish with pasta instead of chicken