Hang with the unruly bunch.
Look for the loud person.
Film faces closely.
Say this on camera: "Can I film you for a documentary?"
If the action is heavy and there's no time to ask, film anyway.
If you think you're about to be pushed or yelled at - ignore this tension in your gut and just continue filming.
Only stop when somebody says stop (and even then...)
Seek this tension constantly while you're filming.
Tension is an indicator that you're filming something interesting, cinematic, thrilling to watch.
So hang out by the ambulance.
Film a cop.
Integrate yourself into a traveling group of people.
Follow a lost person.
Film the dancing loner.
Sit next to a couple of gossiping drunks.
Seek the tension of the environment you're in.
These are the moments that create exciting documentary cinema.