The first half hour of 21 Grams gives you the impression that the production team scribbled the contents of each scene on a separate index card, dropped the lot on the floor, then decided to plot the movie according to the order they gathered them up in.
In fact, the scrambled scenes were eventually shot in chronological order, and as this fragmented narrative is a presumably a feature of his collaborator Guillermo Arriaga's original DF-based novel, a considerable amount of plot straightening must have been required before the cameras rolled. No doubt this helped the actors understand the sequencing and the development of their characters better.
The first thing most reviews of this labyrinthine movie do is question the relationship between style and content. Some have suggested that Alejandro González Iñárritu (a Mexican ex-DJ with a Basque surname) would have been better off to leave Arriaga's plot in its straightened-out format. (Ebert: "You wonder if Iñárritu took the long way round, running up mileage on his storyteller's taxi meter.")
My own view is that generally you shouldn't mess with standard chronology unless you have something interesting to say about the nature of time. However, there was a motor accident at the intersection of the various story strands in Iñárritu's debut film Amores Perros as well, so perhaps it would be unfair to write off the non-linear technique as a showy stylistic quirk. In such stories characters and events orbit a high gravity incident that lies at the centre of their fictional universe. This contrasts with stories which rely on an "inciting incident" in Act One to set them in motion. There are also a number of important precedents in Latin American literature (such as Juan Rulfo's Pedro Páramo, which I'm currently negotiating). After all my recent cinematic and literary jaunts I'm starting to yearn after a story with a conventional beginning, middle and end...and in that order!
Like a choppy sea that calms as you approach the harbour, the scene structure of 21 Grams smoothens out in the final third of the movie. By that time the surface style is much less in your face than the outstanding and moving performances of all three main leads, Penn, Watts and del Toro. These people struggle with themes of birth, death and redemption that are often signalled by symmetrical pairs of symbols.
Iñárritu also puts the haunting soundtrack to work as a gelling agent. Indeed musically at least, there is a suggestion of compositional unity not just between the various narrative shards on display here, but also with Amores Perros, which was similarly scored.
We learn at the end that 21g is in fact the weight of one human soul. This important piece of trivia was determined by the highly dubious experimental work conducted by Dr. Duncan MacDougall of Haverhill, Massachusetts in the early twentieth century. MacDougall weighed dying subjects in order to show that the soul had measurable materiality.