Niggles: To be honest, i would still help (unlike the wankers who stand around and do nothing - sorry but not helping your fellow human being cause ur scared of getting into trouble is a weak excuse). There have been articles where the victim have cried for help, but bystanders do nothing.I find it incredulous...
There can be lots of reasons why they are not reacting:
"Why is no one else reacting? Are they shooting some movie, or is this candid camera? If this is real, does the assailant have a gun or a knife? Where is the police or security? Should I call the cops?"
Also, it takes some time from a person walking on the street to transform oneself from "minding my own business la-di-da-di-daa..." => "Huh? WTF is happening?" => "Action mode enabled!". It doesn't happen instantly. Also some people are simply so shocked of suddenly seeing violence, blood and guts that it alone makes them less capable of acting. Not all people are trained security personnel, knowing how to handle the stress etc..
It is hard to say without being there yourself how the incident appeared. I have hard time believing it would be normally about the bystanders just not caring, shrugging their shoulders indifferently.
What irritates me most are those f*cking teenage girls who start screaming for help on the street just because their boyfriend tickles them. If you reacted fast and yelled to the guy to let go (because you thought there is a rape or a murder going on), you'd next have both the guy and his girlfriend kicking your nutsacks. As said, you first want to understand what is really happening, before you react.
But yeah, I have reacted to some things I've seen outside, like seeing from inside the bus that some big drunkard was beating up a smaller drunkard, so I called the cops. I don't know if many others did too. The stupid thing is that I first called the emergency number (112, similar to 911 in US), and they said there I should call the cops instead. Why the heck they couldn't transfer my call to cops then? I had to ask them "Ok, so what is the number to police then?", and call them separately.
Then again, a few days ago I heard some muffled shouting downstairs. I didn't go on their door to pound the door, or even called the cops. Should I have done that? Hard to say. Similarly I don't run outside instantly whenever I hear one shout outside.
Thunderstone: Quite often under pressure people tend to freeze up, I'm not sure whether or not I'd be able to intervene.
One example that has been used that if there is a drown victim on a beach, some people rush to help (if they have any idea how to help), some freeze (without knowing what to do), and some even run away, trying to block the horrific image (of someone dying) from their mind. By their nature, people are different and react differently. However, I do not think there are bystanders who are just being indifferent, as if they didn't care.
Naturally people can train themselves to such stress situations. So a medic doesn't freeze when giving mouth to mouth to a drunkard, or a security personnel in a restaurant have lots of experience how to react to crazy people and troublemakers. For untrained people, the reaction can even go overboard, ie. when you go to help someone, you yourself become so crazy from the situation and the adrenaline that your "helping" goes a bit overboard. It is not as easy to keep one's cool in stressful confrontations as kung fu movies suggest.