It's hard to say until you're in a situation.
I remember being about 20, and going to a local event to watch an eclipse and walking back late at night with a friend.
A woman was at a phone box and she waved us over and asked if we could wait with her, she'd left her house in nightclothes and called a relative for help, but was worried her husband was looking for her and might find her before the relative came to help her. She wanted us to wait with her and we did. The husband didn't find us thankfully. This was before everybody had a mobile phone and we were prepared to get between him and her if need be to try and keep her safe until her relatives arrived.
Years later I saw a couple arguing in the street and he was aggressive and kind of grabbing at her and dragging her. I had DS in a pram and felt I couldn't physically step in because of him but I rang the police and described what was happening until they got there to help her. In that time lots of people had walked by them and ignored them, he had pushed her against a wall and raised his hand like he was going to hit her, and a man on a bike had stopped to intervene and been punched and so rode away again. As the police arrived the aggressive man ran into the road and was almost hit by a van, which he then started kicking and punching in temper.
And more recently I saw a couple walking down a lonely road, she was either very drunk or very drugged and couldn't walk unaided, he was holding her upright and dragging her along.
If he had let go she would have been on the floor and not moving, she was in that much of a state. I drove passed and something about them bothered me, so I drove back and watched as he pushed her so she was slumped over a wall and then he started shouting, and walking into the middle of the road still shouting. Then he would grab her again, drag her a few more steps and push her back against the wall.
She was kind of dangling over it while he ranted and shouted and stamped up and down.
I rang the police then as well, it was late at night, nobody else came along the road, there were big empty houses with big empty gardens, and I thought better to get the police there than intervene and hope someone else came along to call them.
The police came while I was still on the phone, the woman I was speaking to said it was because I said the woman didn't look like she could walk unaided but didn't seem to want to be with the man, but she couldn't get away.
They arrested him while I was watching, and an ambulance came for her.
I never saw him hit her but god knows what he might have done to her if he'd gotten her back to wherever he was taking her, or if he'd dragged her into one of those big empty gardens.
Most recently of all, I was at work late at night with one other member of staff, who was only 20 and pregnant, and a customer who was in his 70's, and we were attacked by a man who had recently attacked someone with a hammer.
He was getting angry and aggressive and I pressed the panic button for the police but put myself between him and the pregnant colleague and the customer, my reasoning being he would probably still see even an older man as fair game, and if he was going to hit a woman better to hit the one who wasn't pregnant.
He ran at me with his arm raised to punch me and I didn't move, and I think perhaps it was that that stopped him from hitting me. I've never been so scared of getting punched in my life but if he was going to hit me I was going to watch him do it.