Did you ever want to play a video as screensaver? With KDE this is pretty easy.

1. First, you need a video player. I choosed mpv, but mplayer will also do.
2. Second, you need a video. Store it in a public readable place, for example in /opt/cats.mp4. This is not exactly required in a single-user environment, but since your screensaver will be available to all users it is a nice idea. Of course you could also place it in another directory like /etc/screensaver and/or symlink to the real file. This will make it easier to replace the actual video file.
3. Third, create a custom screensaver service file for KDE:

   /usr/share/kde4/services/ScreenSavers/kcats.desktop

   [Desktop Entry]
   Exec=mpv --loop=inf -ao=null /opt/cats.mp4
   Icon=kscreensaver
   Type=Service
   X-KDE-ServiceTypes=ScreenSaver
   Actions=Setup;InWindow;Root;
   Name=Custom Video
   
   [Desktop Action InWindow]
   Exec=mpv --loop=inf -ao=null --no-keepaspect --wid=%w /opt/cats.mp4
   Name=Display in Specified Window
   NoDisplay=true
   
   [Desktop Action Root]
   Exec=mpv --loop=inf -ao=null --no-keepaspect --wid=0 /opt/cats.mp4
   Name=Display in Root Window
   NoDisplay=true
   
   On an Intel NUC I had also to append '-vo=x11' to the option's list.
   

   The exact location of the ScreenSavers directory may depend on your distribution. If unsure, try to locate it with:

   $ find /usr -type d -name ScreenSavers
   

4. Finally, open the Lock Screen menu from KDE's settings and select your custom screensaver.