It will do no good to Bill Viola’s reputation as one of the most important American artists of his generation to be caught in an unguarded moment after a long day’s filming, skipping down a hill yelping: “The best! That was the best of the best!”

This respectful, unobtrusive documentary shot on-and-off over 12 years, follows Viola as he plans and films his crowning glories: Mary and Martyrs, the two monumental video installations on permanent display at St Paul’s Cathedral (described by the Guardian’s Jonathan Jones as “a blast of hi-tech Caravaggio”).

Like Christianity, Viola is all about the biggies: life, death and suffering. He comes across here as a pleasant man, good-natured and modest for an art-world superstar, though with an unmistakable core of pure, blinkers-on drive. Towards the end, he opens up about a near-death experience at the age of six, and touches briefly on his early career. But lots of questions are left hanging. (The obvious: what are his own spiritual beliefs?)

Still, director Gerald Fox captures some moments to treasure. Viola, his wife and collaborator Kira Perov, and the chancellor of St Paul’s damp-eyed, profoundly moved as they contemplate the newly installed Mary is particularly touching. But in the end, this is perhaps a film for the converted – rather than a beginner’s guide to video art or Bill Viola.