Your subjects should feel like their interactions with you on camera are a sacred space.
If there's anything they don't want in the movie, if there's anything they're worried about getting "out there," they should feel like you will honor & protect their trust.
An opportunist (and let's face it, in the movie business, in a way, we're all opportunists) is someone who storms in, takes what they need, and leaves with the loot regardless of the consequences.
You shouldn't do this for moral reasons AND for business reasons.
It's bad business to be cruel.
It's bad business to be an opportunist.
It's only a matter of time before it catches up.
So how do you make them feel like filming with you is a sacred space?
Get close to the subjects: communicate often, hang out together, get to know each other's families, etc.
And when it finally gets time to roll cameras, there'll be a bond between the two of you.
And that bond will be your spear into the good footage.
If times get really tough for your subjects, or if they decide to share something they've never shared with anyone, they will pour their souls into your lens because they trust you.
They give you the goods because they know you're not there to make them look stupid, or steal their souls.
WIN the trust of your subjects, and they will reward you with amazing footage.