Charlie Sheen wasn't always a millionaire bachelor who is either always drunk, or high on a drug called Charlie Sheen. In fact, he started off as a very low, comedic actor, being featured in ONE scene in the film Ferris Bueller's Day Off.
In this scene, Charlie Sheen's character is called "boy in police station" and he is essentially extremely intoxicated and messed up on all sorts of drugs. In order to prepare for this very MINOR role, Charlie Sheen stayed up for 48 hours straight in order to make himself look like a sloppy mess, and 'wasted'.
If he did that much preparation for this tiny role, we can only imagine how much preparation he did for his role in Two and a Half Men.