How to stop screen from dimming when watching video?

You could use a software like Caffeine to easily achieve your need :

Caffeine is a tiny program that puts an icon in the right side of your menu bar. Click it to prevent your Mac from automatically going to sleep, dimming the screen or starting screen savers. Click it again to go back. Right-click (or ⌘-click) the icon to show the menu.

Jiggler

Jiggler will "jiggle" the mouse transparently to stop dimming or other "sleep" actions. No installation, no ktext loading. Just a little jiggle every 20 sec or so.

I find it absolutely indespensible for watching Hulu movies, videos, or any other action where the screen should stay lit, but I don't want to tinker with the energy saver settings.