I was recently on a 737 from MIA-IAH in the 2nd exit row. The entire row in front of me was empty. The rest of the flight was completely full. The last 2 ladies to board tried to sit in the exit row and the FA came right over with her device and knew they weren’t in the right seats and told the out load that this was a seat that people pay for and please go to assigned seats. That is exactly what she should have done so kudos to the AA FA. Unfortunately, i doubt this happens often.