I know the laws vary considerably between countries... so I don't know if China has laws regarding consent for being filmed. In the US there are laws regarding one's right to privacy and right to control how they are perceived... called publicity rights.
You don't need a person's permission to film them in a public place or event so long as you don't use the footage for commercial purposes.
So for instance if you filmed a young attractive couple walking down the beach holding hands and kissing... if you planned on selling that footage you'd need them to sign a waiver saying they give up their rights.
Let's say you didn't get their permission and they hadn't realized that you had shot footage of them.
You sell the footage to a company that makes condoms. Before long an image of this couple is on every box of condoms this company makes.
The couple sues you because they are not married and belong to a religion that forbids sex before marriage. Holding hands and kissing is ok... but the couple claims they've experienced emotional trauma because they've been treated harshly by their families and community. the implication is why would they agree to be on the packaging of a sex product if they were not having sex?
They win a judgment against you and besides that they also win a judgment against the condom company. That company has to remove all of the packages with their image. This costs the company millions of dollars.
Just when you think things can't get worse, the company sues you for damages, millions of dollars. They claim that you sold them footage and photos under false pretenses.
Here in the US, ignorance of the law is not a defense for breaking it.
This is why I feel professionals should take the time to research and understand the laws of the places where they work.
Here in the US, people will walk up to you and ask you if you filmed them. Many will say that you don't have their permission to use their likeness.
If you are in a public place and filming for personal, educational, or news related purposes, you don't need their permission. But any other reason, you do.