The Mercer House, the house Jim Williams lived in, was originally built for Hugh Weedon Mercer. Hugh Mercer was the great-grandfather of Johnny Mercer, who co-wrote most of the songs included in the movie's soundtrack. The movie's opening shot, by the way, is of Johnny Mercer's gravestone.
For the Christmas party scene at Mercer house, they actually invited a large number of people who had previously been at Jim Williams' parties, including several members of his family. These extras praised the movie party because, not only was the atmosphere correct, but Kevin Spacey perfectly played Jim and how he meandered through his parties.
Most of the characters depicted in the movie were based on real Savannahians, but some details were changed for dramatic effect. Joe Odom was indeed a real person, an ex-lawyer who opened a piano bar with the real-life "Mandy," Nancy Hillis, but Odom and Hillis were never romantically involved, and Odom died of AIDS in 1991. Likewise, the character of John Kelso is loosely based on John Berendt, the author of the book, but he also never had a relationship with the real-life "Mandy" - Berendt is gay.
The English bulldog Uga in the film is actually Uga 5 rather than Uga 4 (former mascot of the University of Georgia). Uga 4, who was the bulldog in the book, died shortly before filming, so his son, Uga 5, took his place. Uga is owned by Sonny Seiler who appears in the film and was the real life attorney for Jim Williams.
The trivia item below may give away important plot points.
The movie, like the book, changes a number of factual details about the case. The shooting did not take place on the night of the Christmas party, but in May. Also, Williams was tried four different times over a period of 10 years. He was convicted in the first two trials and sentenced to life, but the convictions were overturned on appeal. The third trial ended in a mistrial, with the jury deadlocked 11-1 for conviction. The fourth trial took place in Augusta rather than Savannah and Williams was acquitted after just an hour of deliberation. Also, Sonny Seiler did not represent Williams at the first trial, and didn't even come into the case until later.