The thing about cops is: they're working class guys and they don't want to get into any trouble. They don't want to rock the boat (but the younger, gung-ho cops are more dangerous). You never want to lie to them, because they're trained to know when you're lying. You don't offer information. And when you answer questions, you couch your answers in the best possible light. Every cop, regardless of how intelligent or stupid he is, has been trained to know when people lie to them. If you're lying, they know you're lying' they can tell. So don't ever lie to them.
If what you're doing isn't overtly destructive or criminal (theft, or assaulting someone), they look at the situation and go, 'Nobody's really hurt by this. And there's thirty people here dressed up as clowns, if I arrest these guys, I gotta do the paperwork. And it's a bullshit charge that's gonna get thrown out.' So if your group of a hundred people is polite to the police, doesn't give 'em a hard time, and doesn't lie to them, the cops will probably let you go. So don't piss them off by saying, 'Fuck the pigs!' or make them feel small or stupid.
The other thing about cops: they sense fear. If you're afraid of them, they know it. They're trained to sense fear, and they're trained to know if you're lying. So don't be afraid of them, don't lie to them, and you'll have a much better interchange with them. These aren't the guys running the world, they're just getting a paycheck, and their job is to protect rich people's property, because that's what they get paid to do.