If you want to watch a moive in Ubuntu,you might want to disable the screensaver and make the display never asleep.And this quick tutorial will show you how to do this.

First go to System->Preferences->Screensaver.You will see the following screen,un-check “Active screensaver when computer is adle” and “Lock screen when sceensaver is active”.

Then,click on Power Management button at bottom.Under Display,set “Put display to sleep when inactive for” to Never.

Now,the screen will never turn black when you’re watching movies!