As I've previously stated, minifigs are the heart and soul of good vignettes. Often you can use the figs that come straight out of the box, but other times it can be important to make your own. By mixing and matching existing minifig parts, you can come up with the fig you want. One of the best builders in this regard, IMO, is Dunechaser. Using existing elements he has come up with instantly recognizable figures from movies, books, and history. Here are just a few examles of Gandhi, Charlie Chaplin, Jimi Hendrix, Admiral Akbar, Ben Franklin and Steve Zissou. The minifig forum on Lugnet is a good place to discuss minifigs.
When existing minifig elements can't give you the look you want, you can alter them with paint, decals, molding clay or even cutting. The Minifig Customization Network is all about this, but forums on customization also exist at FBTB, Classic-Castle and Lugnet. One of my favorite minifig customizers is Norbert Black. He has a great essay on customizing minifigs here. Here are just a few of his custom figs from anime, comics, and other sources.