How to stop screen from dimming when watching video? After a while being inactive with the mouse or keyboard, the macbook will dim my screen to save battery. I know I can expand the delay before it will dim in the system preferences, but are there any way of automatically disable it when watching video?

It's quite annoying having to touch the mouse every few minutes when watching a movie...
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.