Best advice yet!
Before doing anything, call 911. Even if it's only to cover your ass.
Several years ago a couple were having a violent argument in the street in front of my house. Teenagers, ugh. Long story short, guy is yelling in her face, literally a few inches, she's screaming in terror, he's slapping her, the whole nine yards. So I get in his face and push him away, he's dancing around like a boxer, challenging me to fight, but makes sure he stays out of range.
Cops come amazingly quick, two cars, guy immediately says I jumped him. WTF! GIRL BACKS HIM UP!! YOU STUPID FUCKING LYING BITCH!!!!
Fortunately a couple of neighbours had seen everything, there were multiple 911 calls. They came out to give their side.
Point is, without witnesses I would have been up shit creek.
All those people walking by without doing anything, maybe they have been through the same thing. It's easy to say you'd do this or that from the comfort of your computer. Different thing in real life.
BTW, the cops I mentioned in my earlier post: I was at the courthouse once and met one of them who was there to testify in a domestic. He was pissed off. The woman had just refused to testify against her boyfriend. It was the FIFTH incident between them, and the fifth time she had made a statement and then refused to testify.