Sketch comedy tries to satirize things that are in the news or theaters, or themes that are occurring in modern life. Yet, with digital cameras and YouTube, the satire can be of major big budget movies using actual footage. The creator of the remix has no budget, but steals anything available online. Then the creator uses the remix of the material to make it a satire of the original film. With satire, if it's funny it's not plagiarism.
Nothing can be as unintentionally funny as some remixes that try to show tribute to a film, almost as though someone loving a film can be ridiculous in trying to recreate it. Yet, it may surprise some people to learn that people have filmed episodes of Star Trek in their garage. If they have original scripts and images, then remix it is.
Not everyone loves foreign films, but many people love a film that they have no idea is a remake of a foreign film. A movie called "Insomnia" starring Al Pacino and Robin Williams is a virtual shot-by-shot original-footage remake of the original Norwegian version. Yet, someone else other than the original director directed the remake. However, if a creator buys the rights to do this and remakes it with Hollywood names and big budgets, it is technically not plagiarism. It might lack in imagination, but it is his to remake as he wishes. While it is neither plagiarism nor remix, it is one more way to clarify what is what in the reuse of original film.
If a person has the time and inclination to re-film a Tom Cruise movie line-by-line and shot-by-shot, but with neighborhood pets and voiceovers from his friends, as long as he puts it out there online with no intent of profit, it's all good. This is remix in its truest form, in the sense that it takes complete parts of an original film (the script and shots) and remixes them into a movie starring animals with a voiceover. If the creator did it to sell on DVD or release it to theaters, then there is a problem. It's all a rights issue. If he buys the rights, he has the permission to release it for profit.
Outright copying is plagiarism. If a person copies something and then acts as though he had never heard of or seen the original, that is plagiarism and, worse, ignorance. It's not right and it's not creative. It shows no talent and it can be against copyright laws.