Most of the characters (Al Swearengen, Sol Star, Reverend Smith, the Metz family, et. al., in addition to the more famous Wild Bill Hickock, Calamity Jane, and Jack McCall), have real-life counterparts.
Until the end of Season Two, the character of Ellsworth did not have a first name. When it was decided to give him a first name, the actor portraying the role, 'Jim Beaver', requested that he be given the first name Whitney, after Whitney Ellsworth, producer of Adventures of Superman (1952), whom Beaver knew from his research for a book on the life of "Superman" star George Reeves.
Garret Dillahunt pursued the role of Seth Bullock but Timothy Olyphant was already cast. The only role that was available at the time was of Doc Cochran so Dillahunt auditioned for that. He played the recurring role of Jack McCall in season 1. Dillahunt was then considered for the role of George Hearst in season 2 but it was decided that Hearst would not appear on screen until the season finale. Dillahunt played the recurring role of Hearst's employee, Francis Wolcott.
HBO offered the chance to David Milch to wrap the series in a shorter four season, but he declined to do it on those conditions. However, when Chris Albrecht was asked about it, he said that they also told Milch that HBO would give him a full 12-episode season if it was what he needed to wrap the show. Milch told them he would think about it over a weekend, but the news about the show possibly being cancelled reached the press to such a speed that that conversation never happened and Milch just move on to develop John from Cincinnati.
George Hearst was the father of William Randolph Hearst, the famous newspaperman on whom Citizen Kane (Citizen Kane (1941)) was based, and the great-grandfather of Patricia Hearst. When Hearst tells Merrick that he will start his own newspaper in Deadwood to tell lies for his side, it is a reference to the fact that W. R. Hearst is largely credited with the creation of the concept of "yellow journalism" and the use of his own newspapers to shape and even create political and social opinion and actual events. The most famous example of this was what many historians characterize as W. R. Hearst's whole cloth creation of the Spanish-American War through his newspapers' inflammatory and lucrative headlines.
Despite appearing in 32 out of 36 episodes, Jeffrey Jones (Merrick) was listed as a guest star in the first season. Similarly, Gerald McRaney (George Hurst) received a "special guest star" credit in Season 3, even though he appeared in all 12 episodes of that season.
According to a 2004 Jewish Journal of Greater Los Angeles interview with show creator David Milch, when John Hawkes first met Milch to audition for the role of Sol Star, Hawkes told Milch that he was not actually Jewish (unlike both the real-life Star and the "Deadwood" character). Milch's response was to ask Hawkes, "Have you ever felt shame or sadness or ostracized?" When Hawkes responded, "Every day," Milch told him, "Then you're Jewish."
Certain actors that were eventually cast in the series initially auditioned for different roles. Paula Malcomson (Trixie) initially auditioned to play Alma Garrett and W. Earl Brown (Dan Dority) pursued the role of Jack McCall.
The role of Al Swearengen was originally written for Ed O'Neill. O'Neill screen tested for the role but HBO executives did not want to cast him because of his fame as Al Bundy from Married with Children (1987).
Powers Boothe was originally cast as Al Swearengen. But Boothe fell ill before the pilot was to start filming. Boothe was replaced by Ian McShane and, then, was given the supporting role of Cy Tolliver.
The trivia items below may give away important plot points.
The series shows Seth Bullock and Sol Star witness Wild Bill Hickok's arrival in Deadwood, however in reality, Wild Bill arrived in Deadwood two weeks prior to Bullock. Bullock arrived in Deadwood on August 1st, 1876, the day before Bill was killed by Jack McCall.