To some specific incidents on AA....
For reference, I am 6' with broad shoulders - I fill out a seat pretty well but am always well within the boundaries of my armrests, usually just my thigh touching it.
Had a middle seat in MCE on a red-eye SMF-CLT where I was between 1 POS and 1 even larger POS. I had to roll my shoulders in to make it fit. The FA saw my predicament and waved me to the back - gave me the rear jump seat for the flight. Slept the entire way until we were about to land and I wedged myself back in the seat. I am sure that was against all sorts of policies - but with a sold out flight I had nowhere else to go. I was the easiest variable to change in that equation. She got an AApplause Cert.
Window seat on LAX-ORD hop, and I had the window seat. POS in the middle seat and an average sized woman in the aisle seat. FA came by and let the POS know there were some open double seats in the last 2 rows if they wished to move to the back for more comfort. He gladly accepted and was on his way, even wished his former row mates a nice flight. No fuss, pretty simple.
Have yet to be apart of a onboard Mexican Standoff over seat space encroachment. But I stand in the camp that if you are outside the published AA policy of 1 inch over the armrest, I will request some sort of change to remedy the situation. I don't mind moving, even if it is to a less desirable seat location - just make sure my drink is full. I think most folks can see the difference between 2 semi large folks who happen to be seated next to each other AND someone who is certainly a POS and they willfully neglect to abide by the requirement to buy an extra seat in the effort to save money and make other uncomfortable enough to move (thereby getting the extra seat for free).