Here's my insight(?) on the best way to understand this movie. In Jungian psychology, there's a process of meditation called Active Imagination. When applied to Christian meditation, it means imagining yourself in the situation you're reading about. You read the pericope, then you sit back, close your eyes, and try to visualize the situation, and make it come to life. The text is your starting point, but you are not limited to what is in the text. The goal is to 'flesh out' the text, projecting yourself into its Sitz im Leben.I've not seen the film yet myself, but from what I've read and heard, this sounds like a pretty compelling way of looking at it. It sheds light on Gibson's reticence to have named academic advisors for the film.
Gibson comes from a traditional Catholic background. Therefore his texts for the Passion are not just the Gospels as we have them, but the Stations of the Cross, and the Pieta. If you were to do a survey of traditionalist Catholic churches, I'll bet you'd find a Pieta in most of them (there's one in the Church of the Blessed Virgin Mary a few blocks down the street from the Episcopal Church that I attend), as well as Stations of the Cross-- at least during Lent. As I think of the movie, I think every one of the Stations is represented. Although there is some variation about the stations, perhaps the most popular traditional Catholic version is that of St Alphonsus de Liguori (1696-1787), with 14 stations (For an illustrated online version, see http://www.catholictradition.org/stations.htm.) The importance of this for present purposes is that 5 of these 14 are non-scriptural (i.e., not to be found in the canonical Gospels), and Gibson has all 5. The other 9 are taken from, or implied by, passages from all 4 canonical Gospels (e.g., John 20:25 is used to imply that he was nailed to the Cross). Michelangelo's Pieta is not one of the Stations, but its place in Catholic Tradition is so great that any visualization of the Passion can scarcely ignore it.
In addition to the Stations and the Pieta, the whole thing is framed by a quote at the beginning of the movie from one of Isaiah's Servant Songs, 53 . . .
. . . . . So the basic script for the movie, IMHO, was set by Isaiah 53, the Stations of the Cross, and the Pieta, fleshed out by Gibson's own "active imagination." In his active traditionalist imagination, Romans speak Latin (isn't that what they're taught in school?), so that's what they speak in the movie. The quote from Isaiah at the beginning tells us that it is irrelevant whether the Jews or the Romans were to blame; Jesus' suffering was required by the doctrine of Atonement, and since our (collective) sin is so great, his suffering had to be great enough to match. This accounts for the gratuitous extra images of suffering, such as the Roman soldier who pulls Jesus' shoulder out of its socket in order to stretch his hand out before nailing it to the cross, which are part of Gibson's act of Active Imagination."
Update: see this article on Beliefnet for more on the relationship of the film to the stations of the cross:
What's Catholic About 'The Passion'? A Lot
The Stations of the Cross, the Sorrowful Mysteries of the Rosary, and Catholic mystics' visions shape Mel Gibson's work.
By Jennifer Waters