Legal advice (posting on YouTube)?

If you aren't the one who originally held the camera, then it is copyright infringement. You cannot duplicate or reproduce even a second of somebody else's work, particularly not that of a major motion picture, without violating the law. You must cease and desist all such activity immediately and remove any videos from YouTube that contain even moment or even flash of any video that you yourself didn't film, even if that video is just playing on a television set in the background, you can't use it.

There is a provision in the law that allows people who are publishing film reviews to use copyrighted materials of the film's original production. That principle of the law is called "fair use." However, if you do not have a team of lawyers at your disposal to help you navigate what is and isn't fair use, then you had better steer clear of using any material at all. It is such a tricky law that major news outlets and publishing companies have gotten caught up in court fighting lawsuits about the definition of fair use.

You re talking about making a derivative work, which still requires the permission of the copyright holders involved.

It's illegal copyright infringement.

It would be illegal.