all is here : you can't "play" with the darkside, you can't switch between the two sides as you want. (except of course for a final redemption) This is one of the biggest PT mistake because it contradicts everything that is said about the dark side in the original trilogy. Falling to the dark side is not a matter of weeks, months or years...A jedi can fall very quickly, in a very short time, and then he can't come back...that's what makes it so dangerous. In the prequels, Anakin do what he want, anger, hatred, murder, and then go back to his asshole "good" behavior, it makes no sense. Everything about the dark side is wonderfully done in the original trilogy : everyone understand that a Jedi, because of his power, can't do what he want, he can't have a normal human behavior, he must perfectly control himself because one misstep can be fatal. The final confrontation in the Return of the Jedi showed this perfectly : Luke use his anger to defeat Vader...but, in the border line, he realised his mistake and control himself...but he was on the brink to fall for only a few moments past in the dark side.