If you want to disable X screen blanking (aka. disable the X screensaver) that comes up unexpectedly while you’re watching a movie in MPlayer, all you have to do is
setterm -powersave off -blank 0
To make the change permanent, add the following to your ~/.xinitrc file:
setterm -blank 0 -powersave off -powerdown 0
xset s off
It’s better to file a bug against the movie player (even better, include a patch!).
If you disable power saving altogether, you are wasting electricity, and also may risk burning patterns into your monitor – even LCD screens will burn eventually.
You could rename mplayer to mplayer.original and make a shell-script wrapper, e.g.
#! /bin/sh
xset s off
mplayer.original “$@”
xset s on