I'm wondering what would have happened if one of the passengers who was witnessing the United incident unfold had stepped in and tried to defend the doctor, by force.
Note that I'm assuming the circumstances are exactly as they played out here; for example, the officer (if he was really a police officer) is not obviously dressed as one, so you'd have to make your own inference as to whether he was a police officer.
Specifically, I'm wondering what would happen if someone saw the doctor was being visibly injured and you punch/kick/etc. the officer to try to separate the two, in defense of the doctor. I imagine you'd be arrested and in jail until your trial, but I'm wondering what would happen afterward. Is this likely to be considered a defensive move and could you be found not guilty if you were taken to court? And if you're taken to court and found guilty, what would the consequences be like -- a few days behind bars maybe, or a years-long prison sentence?
Assume you're not trying to permanently injure the officer here, just some temporary/minor injuries so he gets the message. The goal is obviously to defend the other guy with a bit of force, not to attack the officer and make him fear for his life. So by the time the court date rolls around, he'll at most have a bruise, if anything.
(Editorial note: Yes, the better passenger-initiated solution would've been for someone else to give up their seat, but if for whatever reason you didn't do that and this was the only option left...)