If an individual is asked to contribute to a video, for example by playing a (leading) role in it, and agrees, they thereby give permission to use their image in the video. However, it is advisable to have this permission in writing for all the video's intended uses to avoid any subsequent objections, for example based on an individual's right to control the use of their identity and the GDPR protecting individuals' privacy.
Non-professionals
Even if the contributors are not professional actors but extras or people who are being interviewed or demonstrating something, you are advised to have them sign a contributor declaration (or quitclaim) in which they give you permission for all intended uses of the video.
Passers-by
There are also those who appear on camera by chance, such as passers-by during a street interview. It is impossible to ask all of them for their permission. According to these individuals' right to control the commercial use of their image and likeness, they can only object to the disclosure of the video (or photo) if they have a reasonable interest in doing so. Also consider the privacy aspect: someone may be filmed in a compromising position or situation. And the financial aspect: celebrities may ask compensation for appearing in your production.
Patients
Special privacy protection rules apply to patients.