As someone who has worked with religious films series and viewed many biblical films, I view the fact that most Gospel films draw on more than one Gospel account and sometimes non-biblical sources as a problem for the artistic and religious value of these films. The harmonization of the Jesus story in film has produced bland Jesus's, mediocre story telling and theological error. Most of the controversy in Gibson's "The Passion" was caused by the inclusion of material from the visions of mystics. Zefferilli's "Jesus of Nazareth" comes closer to a successful harmonization, but it portrays Judas as duped into betraying Jesus. Theological errors creep into such films simply because the directors and script writers are not viewing the Gospels as each being a unique authority, and because there are seemingly contradictory elements in each account that represent parts of the unique message of them.
In contrast, some Gospel Films such as John Heyman's "Jesus" (1979) and Pier Passolini's "The Gospel According to St. Matthew" avoid this problem by choosing one account. I haven't seen Heyman's film, but I'll include some of it of it from You Tube. Passolini's film, by contrast, I have seen several times. Passolini was a Marxist, an atheist and a homosexual, and yet his film was, up until Heyman's movie, the most faithful Gospel film. He included absolutely no language not found in Matthew's story. His photographic staging did, however, inevitably color the story somewhat with his belief in Jesus as figure in class struggle, but it was otherwise completely faithful-- to the point where the hard sayings of Jesus hit home and trouble some viewers.
There are other films, such as Scorsese's "The Last Temptation of Christ" depart even further. It is based on the novel of the same name by Kazantzakis, a Greek writer of Gnostic tendency who used his imagination, and not the Gospels, as his primary source. The sexuality of the novel becomes even more problematic in full splendor on the screen. While even in this film there is a distorted reflection of the gospel Christ, it is faithful to nothing the Gospel's teach.
When a Gospel film uses multiple texts, perhaps even non-Gospel sources and the imagination of writers, however vivid and creative, they stray from the essential function of a Gospel film religiously and artistically. A mislocation of the function of the art in this context has occurred. It goes back to the ancient purpose of religious painting as defined in the early debates about Icons. In the early church some wanted no depiction of God or the saints for fear that idols where being produced. Besides the matters of who was being depicted, and for what purpose, there was another defense of icons offered. Not only were these depictions of God and his saints as opposed to pagan gods, and objects to promote the Christian faith, but they were designed to refer away from themselves. Icons were not painted to draw one into them as objects of worship or devotion in themselves, but rather were made to point away, towards God in heaven, towards the mysterious and unknowable God. In that sense, Passollini's film perhaps becomes the closest thing to an icon in film.
Diatessaron - Wikipedia,
The Problem of the Cinamatic Jesus
Icon - Wikipedia