According to Brett Martin's 2013 book Difficult Men: Behind the Scenes of a Creative Revolution: From The Sopranos and The Wire to Mad Men and Breaking Bad, HBO only gave Alan Ball one note (criticism) on the pilot episode, which was: "we love the characters. We love the story, but the whole thing feels a little safe, Can it be more fucked up?" In response, the episode three sequence in which Claire steals a foot from the morgue was written.
Michael C. Hall's first screen role. He had confined himself to the New York stage prior to "Six Feet Under". The first scene he shot was in the pilot episode when David goes to the morgue to collect his father's body.
Nikolai's flower shop actually is a real flower shop at 14325 Ventura Blvd and was once a gas station. It was in fact the gas station that James Dean filled up at the day he left out of town on his way to the crash that would kill him (there is a color photo of him at the station about to get back behind the wheel in fact).
Rachel Griffiths had read the script and had indicated to the production that she was very interested in playing Brenda. The producers' only real concern was whether she could pull off a convincing American accent. When Griffiths flew over from Australia to meet them, she arrived complete with perfect accent, and got the part.
Both of the main male leads in this show ('Peter Krause' and Michael C. Hall) subsequently starred on other TV shows during which they dated actresses playing their sisters. When Krause played Adam Braverman on Parenthood, he started dating his costar Lauren Graham, who played his sister Sarah Braverman. When Hall starred in Dexter, he started dating (and later married and divorced) his costar Jennifer Carpenter, who played his sister Debra Morgan.
Peter Krause (Nate Fisher) originally auditioned for the part of David, as he was impressed by the political/human rights message that the role had and he wanted to stand up for the character. However, the creator, Alan Ball had found the role of Nate Fisher impossible to cast, and was impressed by Krause and his tangible chemistry with Rachel Griffiths (Brenda Chenowith).
The difference in ages among the Fisher siblings differs somewhat from that of the actors who portrayed them: Nate was born in 1965, like actor Peter Krause; but David was born in 1968, while Michael C. Hall was born in 1971; Claire was born in 1983, while Lauren Ambrose was born in 1978. So, while there's a 14/18-year hiatus between the births of the brothers and the sister as characters (and it's relevant to the story), the actors' actual birth dates are roughly equidistant.
The initial inspiration for this show came from Carolyn Strauss, then the president of HBO's entertainment division, who, shortly before meeting with showrunner Alan Ball, had watched the 1965 film adaptation of the 1945 novel The Loved One, a satirical expose of the funeral industry. Ball's further inspirations for the show included Jessica Mitford's book The American Way of Death and Thomas Lynch's essay collection The Undertaking: Life Studies from the Dismal Trade.