I’d like to think that I’d step in to help, but it would have to depend on the environment, the people involved, time of day, who I’m with etc, etc.
A friend of mine once stepped in to stop a girl being attached by her ‘boyfriend’ and was stabbed in the back of the leg by the girl he was trying to help (although I think this was probably because he was a little heavy handed)!
I was once walking home from the pub and heard a cry for help from a dark street and I had a real moral dilemma as to whether I should do anything, in the end I decided to investigate and found a guy who needed help (he’d recently had an operation and had fallen) so I helped him get home (and felt good for doing what in hindsight was the right thing), but it was pretty scary as it could have easily been a trap to rob me!