Anatomy of a Sucker Punch

Sucker Punch

This is a good example of poor situational awareness. If the victim had made eye contact or acknowledged the thug in some way he may have appeared to be a "harder" target, or at least a more troublesome target. watch the thug look over his left shoulder and sway back just before the attack.

Preempitive Strike
It looks like the guy in green wants a fight, watch how black jacket sets up distance, closes and strikes. Watch how black jacket softly controls arms at close range. he has already set up a defensive stance while keeping a non aggressive posture. Watch closely for left arm control while right fist launches.

Payback
Good defensive example. Note the street fighter "kata". Black shirt should have his hands up sooner. He waits until thug grabs his neck. Watch for the braichal stun, it doesn't work to great effect because the thug has not committed himself to the attack. Finally he sets up a low fence and finishes with a hard straight left.
PS this is not the US, or the self defense would have started much sooner.