Sometimes I manage to catch on quickly enough to defend somebody else; but the biggest problem for me is that social interaction is so quick and I can't figure out who's trying to do what to whom while intervention would still be useful.
If it were really obvious, like somebody was physically attacking somebody else, I'd probably be able to figure it out quickly enough to act. Unfortunately, in that case it might have to be either calling someone in authority or physically getting in between them and hoping they would stop punching before I got too hurt. I can't fight; I'm not fast enough, and I'm very clumsy. But it's still hard to shift two hundred pounds of determined woman when she doesn't want you to do it. It depends on whether they seemed intent on hurting each other, or just intimidating and bullying. If it were dangerous enough I might just call the police, or pepper-spray somebody.
What I learned about altruism in psychology class is that in order to help somebody, you need three things: One, you need to interpret the situation as dangerous and requiring your help. Two, you need to see it as necessary to help--that is, if you don't help, nobody will. Three, you need to be able to think quickly enough to create a plan of action so that you are not too confused or afraid to act.
It's hard for me to fulfill those criteria on the spot. I have no problem with #2 because I never assume other people will help, because I never really take them into account (probably due to autism--it's too complicated mentally to always be tracking the hypothetical decisions of the people around me). But the first and third requirements are tough. Being able to recognize a situation as one requiring your help, and being able to make a plan before the novelty of the situation causes you to freeze or shut down, is a difficult problem. It takes a lot of thought, a lot of multi-tasking. The only reason I can ever do it is that when I really, really need to, I go into emergency mode, draw on adrenaline, turn on everything, flood myself with sensation--it's exhausting and I will absolutely crash and shut down afterwards, but if I need to, I can. I used to function like that routinely; but I know better than that now. Now, I only use it when I really need to.