IT IS a nebulous concept, authenticity. Especially when it comes to dramatising a character whose image has been constructed and reconstructed over the centuries, shapeshifting to suit whoever is telling the story. Take Joan of Arc. The 15th century saint has been variously claimed, or vilified, as a feminist, Catholic martyr, resistance fighter, French patriot, cross-dresser, pixie cut icon, political emblem of the left and of the right. For anyone who has seen Danish director Carl Theodor Dreyer’s 1928 silent film masterpiece La Passion de Jeanne d’Arc, only one image of Joan will forever be burned into their retinas: those steely cheekbones and saucer eyes of Renee Maria Falconetti.

So expressive is Falconetti’s face that the performance is still routinely hailed as among the greatest of cinema history – a "symphony of faces". What makes the film so good is its uncompromising intensity, its austerity and weird camera angles, its frenetic cuts – and, especially, its excruciatingly unwavering close-ups that glue us to Falconetti’s face. The effect is unapologetically visceral. We feel Joan’s pain. As we sit and watch, we also feel complicit in her execution.

Dreyer was a fierce director. He was famously tough on his actors, pushing them to the point of real blood and tears. He didn’t permit any makeup, instead wanting emotions to be conveyed through the rawest of facial expressions and ultra high-contrast lighting that would show up everything, warts and all. But here’s something that seems completely implausible nowadays: he had no control whatsoever over the make-or-break factor of music.

In the 1920s, films were screened to the accompaniment of nothing or anything at all: a local pianist or a symphony orchestra, depending on the size and prestige of the cinema. The precise score was liable to change on the night, especially when the soundtrack was improvised, which it often was. The notion of having carefully planned music that would reflect, enhance and counterpart the screen drama? That came later.

Actually, there was original music created for the Paris premiere of La Passion de Jeanne d’Arc, scored for chorus and orchestra by a couple of operetta composers (Victor Alix and Léo Pouget; no, me neither). Dreyer had no choice in the matter. That 1928 score has been revived from time to time since, but nobody has suggested it comes close to doing justice to the film. Over the decades there have been multiple efforts to match La Passion de Jeanne d’Arc with the right music, from grand oratorios to ambient electronica. Nick Cave and the Dirty Three played a live score at the National Film Theatre in London in 1995; four years later, Cat Power tried a similar gig in the States.

Dreyer died in 1968 without having heard any score that he felt really worked. In 1952, the Italian producer and critic Joseph-Marie Lo Ducaput screened La Passion de Jeanne d’Arc with a soundtrack of baroque music, going for a vague period-ish feel without bothering to get the right period. Dreyer hated it – primarily because Ducapot had trashed the film’s meticulous framings by cropping the image to make room for a sound strip. When it came to Ducapot’s musical choices (Bach, Vivaldi, Albinoni), why, Dreyer wanted to know, hadn’t he opted for actual music of the actual 15th century?

One simple answer is that even by the 1950s we were still pretty much in the dark about medieval music, yet to benefit from the great early music revolution that would happen in the 1960s and 1970s. When Ducapot made his soundtrack, 15th century manuscripts were still only circulated in very niche corners of the music world, and there was hardly anyone who would have known how to sing this repertoire properly.

Queue medieval vocal music specialists Donald Greig and the Orlando Consort, who, 60 years later, have finally crafted a score using the music of Joan’s time. The match is perfect, Greig tells me: “the film is set in the early 15th century; it’s about religion; we can directly illustrate the moments when the liturgy is being celebrated; we can directly mirror that sacred space, that sacred dimension. This is the kind of music that Joan would have heard herself.”

Most of what Greig has selected is French church music, with the exception of the English Agincourt Carol. On-screen action is synched to what we hear – and it’s not all smooth, long-lined polyphony. “That would be too rich,” Greig explains. “We need a bit of sorbet. There are some scenes where I wanted to illustrate Joan’s sadness, so I choose a melancholy chanson. Elsewhere I’ve tried to reflect the atmosphere of hectoring priests or torture instruments. At some points I’ve gone for something quite illustrative in the text – an Ave Verum Corpus [a Eucharist hymn describing the piercing of Christ’s side] while we watch Joan bleed.”

The effect is potent. Tracking shots around the courtroom of Rouen are accompanied by plainchant; the covert scheming of Joan’s judges is matched by cacophonous whispers; the moment when a persecutor spits in Joan’s face is punctuated by harsh Latin consonants. Sometimes the Orlando singers simply stop singing altogether, augmenting the toughness of the film by making the audience really feel its silence. “That is its brilliant paradox,” says Greig. “It’s a silent film about voices. Joan hears voices all the time. She’s surrounded by false accusations. Yet she herself is voiceless.”

Greig has a background in film studies, and he explains why Dreyer’s radical editing techniques have been such a magnet for so many musicians. “The average length of a shot is just over three seconds,” he says, “which is incredibly rapid and quite aggressive. These contrast with endless slow close-ups of Joan’s face. There’s a fascinating rhythm to it – in fact, the film itself is tremendously musical.”

The Orlando Consort have been touring their live score internationally since 2015, and responses, says Greig, have been immense. For him, this project marks something of a personal victory.

“I’ve coerced my musical colleagues into watching this silent art house film,” he laughs, “and on the other hand I’ve convinced promoters to put on a whole bunch of early 15th century French vocal music that would never otherwise sell.”

When the project comes to the Lammermuir Festival this weekend, it will take place in St Mary’s, Haddington – a medieval church that was was consecrated around the same time as Joan’s life. Big screen aside, it doesn’t get much more authentic than this.

Voices Appeared: The Orlando Consort’s live soundtrack to La Passion de Jeanne d’Arc is at St Mary’s Church, Haddington, on September 17, part of the Lammermuir Festival