How did mark twain create his characters?

I believe this varies from book to book, but in some cases, Twain drew heavily upon people he knew, either in childhood or adulthood.  Shortly around the time Tom Sawyer was published in 1876, Twain asserted that Tom Sawyer was based upon 3 boys he had known from childhood.  Later Twain claimed he was the inspiration for the character.  A Guardian article from 2010 indicates that Twain actually based Tom Sawyer on a real life person he met in passing, a hard-drinking firefighter from San Francisco.