I don't usually watch movies from my laptop, but when I do, I don't want the screen saver to kick in.
Occasionally, I notice that my old time movie player mplayer does not inhibit the screen saver or screen blanking. That means that 5 minutes into a movie, the screen turns black.
I don't think this used to be the case. Maybe it's because I'm running the Wayland display server in GNOME now.
In any case, inhibiting the screen blanking can be done using the gnome-session-inhibit program:
gnome-session-inhibit --inhibit idle PROGRAM
gnome-session-inhibit --inhibit idle mplayer /path/to/wholesome-documentary.mkv