I'm not very good at confrontation - and as far as my physical stature, it doesn't exactly lend me confidence to be assertive (verbally or physically) to "walk the talk" - and I don't bluff.
If it were an act of violence, and it was between clearly unequally built/skilled folks, I'd - I think I'd yell to get others' attention, dial 911 on my cell, and yes - run toward the confrontation and try to de-escalate it.
If it were a property thing - I'd call the cops. If the desk officer/operator asked me to remain where I was so that I could assist in I.D. or describing and what not - I'd do it. I did this in college. Two obviously drunk students were kicking the crap out of somebody's car - I was on the 5th floor in a piano practice room. Yes - I'm a night owl - and this was common for me.
I did testify at the trial, too - but other than confirming that the crime took place, when, where, and the words spoken, I wasn't of much help given that the prosecutor wanted me to describe the two. At that time I thought it was just an eyesight issue, but I also have a lot of trouble processing, interpretting, and remembering anything visual - ESPECIALLY non-geometric or unlabelled/unnamed aspects of the visual realm. But I did the best I could.
Now, if somebody were hurting a kid, I wouldn't bother to call 911 right away. Yes I'd yell for him/her/it to stop. And if it was still going on as I arrived - I would physically step in and yes - I would restrain the aggressor as much as I could.
I'd like to believe I'd have the kajones to jump in and kick the person's face in for beating up on a kid. But I know I wouldn't. But get in the guy's way - even if his beating turns to me - yeah - I know I'd do that in a heartbeat.
Then I'd spit on him as I walked past him on my way to the witness stand. ;)