Robert McCallister was the name of a character on a previous Greg Berlanti show, "Jack & Bobby" (2004) about a boy that grew up to be the President of the United States. As with Brothers & Sisters, he had a wife named Courtney and a son called Jack.
While the producers were looking for an actor to play the role of Jason McCallister (Robert McCallister's brother and Kevin Walker's love interest), Rob Lowe, who plays Robert McCallister, recommended John Barrowman, who had costarred with Lowe in "A Few Good Men" on the London stage. The producers offered the role to Barrowman, and he was interested in taking it, but was ultimately unable to work the shooting schedule around the schedule for his own show, "Torchwood" (2006).
Unusually for a show that turned out to be a critical and commercial hit, in the weeks before the first episode aired, this show went through several cast and crew changes. The Walker family matriarch, Iva, played by Betty Buckley in the original pilot, was recast with Sally Field and renamed Nora. Walker brother Bryan, played by Jonathan LaPaglia, was recast with Matthew Rhys, renamed Kevin, and given a new back story (instead of Bryan being an ex-husband and father who was newly out of the closet, Kevin was a never-married lawyer who was more comfortable with his homosexuality). Marti Noxon was replaced as the showrunner by Greg Berlanti because of "creative differences."