I hate people who just stand there and scream "Somebody help them". You. You go and help them. If you see someone needs help and you are there. You help them. Screaming for someone else to do it comes across as the most self serving shit in the world. It's like those idiots you hear shouting "Someone call the police!" when they're watching a crime happen. No, you do it. You just want someone else to do it so you can watch the show but still claim that you helped.
That's the bystander effect. People are terrified of doing the wrong thing or getting sued for doing the right thing. Often they also have no idea what to actually do to help beyond vaguely begging the air around them to "make it stop."
The most effective thing you can do to break the spell is to start barking orders at specific people. Instead of just yelling at the crowd "someone call the police," you approach one specific person (anyone will do), make direct eye contact with them, point to them and say "you! Call 911 right now, tell them to send police and paramedics." Don't give them a chance to argue or refuse -- immediately move on to the next (randomly selected) person and get them moving on something else that's relevant to the event.
Make eye contact, point to them and say "run inside the store and ask the clerk for water and clean wash cloths" or "go put out warning cones, flares, triangles, anything you can out in front of the crash to warn other cars away from it, then do your best to direct traffic around it until the police come" or "go find a first aid kit and bring it back here" or "go get those people to back away a bit to make room for paramedics." You don't have to be incredibly specific and you shouldn't try to act in an "official" capacity if you're not an emergency responder or otherwise qualified to do so. You're just organizing people to help rather than just standing around holding up cell phones.
Most people will comply and get to work. People are generally willing to help in a lot of situations, but don't know what they need to do and also don't have any (initial) confidence they are "qualified" to help. Most importantly, they're afraid of making decisions in a crisis. Once you (or someone else) steps up and starts making those decisions for them, they're much more likely to help since someone now they're being told what to do instead of being silently expected to just "figure it out and help."
A handful will balk and either tell you to fuck off or just ignore your instructions and continue gawking, but in either case you just move on to the next person and repeat the instructions. You can throw in a quick "what's wrong with you? DO IT!" if you want to push back against a "rebel," but it's usually better just to write them off and move on. People like that tend to vanish quickly once everyone starts pitching in and pressure mounts to contribute instead of watching from a distance.
Once someone "official" arrives, they'll probably approach you directly since it'll probably look like you're in charge. If they don't, approach them yourself and tell them what's happened so far and what people are doing. Then hand the reigns over to them and then offer to help in whatever way they need.
