Script Magazine has an archived article from 2004 with screenwriter Guillermo Arriaga on the process of writing the film '21 Grams'. It covers the initial nugget of the idea, themes, characters and the film's big questions.

"We all know that one scene has a meaning by itself and a completely different one when it is linked to a scene before and a scene after. I wanted to go beyond the conventional, so I went for symbolic choices. I started writing scenes in a contrasting order, so that the structure itself would help change the audience’s perception of the story, making it much more emotional. That was my main goal: Make the story emotional, not just intellectual. I wanted the screenplay to challenge the heart as well as the mind. I’m not interested in creating jigsaw puzzles. I like structures that help the audience get involved in the emotion of the story, so they can smell, touch, see, feel. That’s it: feel."