Last year I read this interview -- He's traded safety of sitcoms for the danger of 'Dexter' -- with Clyde Phillips, a producer for the show Dexter, and was stuck by what he had to say about emotion, character, plot and flashbacks.
Q. Does the show have any internal rules?
A. We try to tell the stories though Dexter's point of view. We only use slow motion if we're either in Dexter's head or looking at him. The flashbacks always have to be emotionally motivated. Almost all of the flashbacks are back to Harry, his father. They need to be emotionally driven, rather than plot driven. I think all of that contributes to the success of the show. It's emotion, character, and plot, in that order. And I think that's why people embrace the show.
I've since watched the first season of Dexter on DVD and I was surprised with how much I liked the show. Dexter is an extremely compelling and sympathetic character, even though he is a serial killer. And the flashbacks, which can be a clunky storytelling device when used poorly, are vitally important to understanding Dexter as a person.
Sometimes when a storyteller uses flashbacks, the writer is just being lazy. Dexter is an example of flashbacks used with purpose and effect.