If you watch a movie on YouTube, can you be sued for copyright infringement?

Not you, but the person who posted it.

@diavolobella Exactly. Indeed, many videos have been taken off the net for copyright infringement.

Though I am not sure what can be considered fair use. Like, it is probably copyright infringement to post an official Katy Perry music video on Youtube, but what if you take parts of the song and make a music video that is composed of original images (i.e. images that you created with a camera, paintbrush, software, etc.)

Even taking parts of a movie or song and making a video of your own original images can get you into trouble, if the artist chose to pursue it. The odds are they would not, because “fan videos” are a source of goodwill for the artist and, as long as you aren’t making money from it, they don’t want to alienate a fan.

That said, to fall under the category of “fair use” there are several criteria that must be met. The use of the work would need to be for a legitimate purpose that is not designed to make money, such as non-profit educational purposes or by a critic while giving a review. It also would depend upon how much of the work you used. Using a film clip or a snippet of song is vastly different from copying the entire work. The last thing to be considered under the Fair Use Doctrine is whether or not the person in question’s use of the artist’s work will damage the artist’s ability to make a profit from it. Lastly, acknowledging the song or movie is another person’s work and not yours (“this video I made of Katy Perry’s song is my homage to her) is not a substitute for getting permission from the artist. You see people make that sort of disclaimer on Youtube all the time, but it wouldn’t cut it in a court of law.

If you re-posted it you could, but not for just watching it.