If the PC does not detect any human action for a given time then there are different behaviors possible:
Do not react at all and keep screen active
Blank the screen and some monitors detect that and go in power down mode. This is called DPMS and is triggered by the kernel to save energy
Start a screen server as xscreensaver
It is quite annoying when this happens repeatably while watching a movie. There are some solutions for that:
Turn off DPMS using the xset command as xset -dpms or xset s off
Install a screensaver and use a movie player that can talk to it so it does not trigger. Or just set a time that is much longer than the movie.