Johnny Flynn on composing for 'As You Like It', Shakespeare's genius, and the charm of the nyckelharpa

It's night-time and there are strangely beautiful noises emanating from the back rooms of Shakespeare's Globe in London. Through a stage door, down a winding passage far beyond the pomp and glamour of the main stage, in a round rehearsal room that by day is one of the stops on the theatre's guided tours, is a group of musicians who have dedicated themselves to learning the idiosyncratic ways of Elizabethan musical instruments.

The musicians are arranged in a circle with Johnny Flynn at its centre. The Shakespearean actor and frontman of the folk band Johnny Flynn & The Sussex Wit is now blending his two passions, music and acting, by composing the music for the Globe's new production of As You Like It, directed by Blanche McIntyre. He's doing it using only period instruments.

Griselda Sanderson, one of the circle of musicians, is one of only a handful of people in the UK who plays the nyckelharpa, a Swedish instrument meaning "key harp". It was first used in the 13th century and became popular in Shakespeare's day. It resembles a pimped-up violin. She plays it expressively with a bow and keeps it close to her body using a customised blue sling around her back, keeping her hands free to dart across the strings and nodules on its neck. Adrian "Woody" Woodward, the music director, sits across from her. He plays a cornet, not the modern brass version but a curved wooden horn covered in black leather. Its popularity grew in the Renaissance era and by the 17th century people believed it had the same expressive qualities as the human voice, and Bach used it in some of his cantatas.

This is all new to Flynn, 32, who can play banjo, guitar, trumpet and violin (the last two of which were mandatory requirements of his music scholarship to Winchester College, the public school he attended) but is not an expert at the nyckelharpa, or the hurdy gurdy, or the hardingfele fiddle, with its lion's head carved above the strings, or any of the antique instruments surrounding him. Flynn composes the music for his indie folk band, which has toured with Laura Marling and Mumford & Sons, and he has written scores for films and television.

But he is finding these curious instruments have their own language. So why is he involved in this project? He is fascinated with the Elizabethans so naturally, eventually his path would lead to Shakespeare, his obsession. He's thoroughly enjoying the challenge. "I think this is the most fun I've ever had because it's so stimulating. All of this production process is part of that world that actors can never see and I've been part of it for the first time," he says. "Actors are very mollycoddled in a way so it's really interesting to get the whole picture."

Flynn is no stranger to performing at Shakespeare's Globe. Most recently, last year he played Lady Anne opposite Wolf Hall's Mark Rylance in a double bill of Richard III and Twelfth Night. Off-stage, they discussed their theories about who Shakespeare was. Rylance believes he was not one man but a consortium of different writers. Flynn says that letting go of the idea of Shakespeare the genius was liberating for Rylance, like "letting go of Jesus".

The main difference between this process and being an actor is that the musicians at the Globe have comparatively short rehearsal times (actors will typically have eight weeks to rehearse for a performance, but musicians are brought in at the end of the process and get just four weeks to practise). Plus, there's none of the glory of being on stage. "I know I'm not going to be in the actual performance, which is actually quite a relief in a weird way but I'm getting to think about the whole story rather than just one part," says Flynn.

Flynn's compositions were written before the musicians had been assembled and, because he hasn't worked with these instruments before, he then had to adapt the songs to accommodate them. "These instruments have idiosyncratic and peculiar notes," explains Woodward.

Flynn says it's been a very collaborative process. "I've had to really study up and learn what the ranges are and so I've written things and then Woody will look at it and say, 'It's really nice but actually I can't play a B on that cornet', or 'I can't breathe, can you leave in some spaces', so I've made changes based on their feedback."

The instruments are divided into two camps: those for Elizabethan street music, which is more wild and free, and those used for the court scenes, which are more formal.

"Musically there are two worlds, the court and the woods, says Flynn. "Everything in the court is really uptight and everybody is really on edge and this tension really comes across. "

He reads out, with relish, Duke Senior's speech as he leaves the court to live in the Forest of Arden, beginning with the lines "Hath not old custom made this life more sweet/ Than that of painted pomp?/ Are not these woods/ More free from peril than the envious court?"

Flynn continues: "So there's a really powerful spirituality to his sense of nature. It's really cool. In the court it's all corseted and then in the woods they've been left more to run wild, and what's beautiful is it's not an easy life in the forest. They keep saying, 'It's cold', or whatever, but the music reflects that painfully exquisite beauty of living that close to the ground, which I really like. I like going on really long walks and putting myself in that position so I recognise what these guys are going through."

Flynn was born in South Africa but his family moved to Hampshire when he was three and he has always had an affinity with the British countryside. He understands the play's dichotomy between wanting to be in the thick of the action but yearning for the freedom of being in nature all too well. "I live in Dalston and I go to Hackney Marshes a lot, and my mum lives in west Wales so I go there a lot, so I try and get out of London as a much as possible, but doing projects here, it's quite hard. As a jobbing actor or musician you have to take any work you can get."

The first piece of music audiences will hear is the Rosalind Celia Theme, which establishes both the court music and the theme for Rosalind and Celia, played with trombone, lyre and nyckelharpa. The group try it with the cornet and then repeat it with recorders. To an untrained ear the difference is subtle but there's something more fluid and airy about the cornets. Woodward says the cornet is usually used to represent the underworld.

As with many of Shakespeare's comedies, As You Like It's finale involves a dance. For the final jig, Flynn has selected a piece called La Volta, which starts in a minor chord with a rousing intro and big marching-band drum. "There's a portrait of Queen Elizabeth being lifted in the air by one of her suitors," explains Flynn, "and she was dancing to La Volta. It's about the different hemispheres that men and women inhabit and about them coming together. There are often syrupy final dances performed at the end of Shakespeare but this is different – it's quite risqué, like a tango; the women are lifted up in the air by the base of their corsets."

Its time to go. Flynn shows me through the labyrinthine passages that lead back out into the normal world, and just before he leaves to return to the studio I ask what his take is on Shakespeare. Does he believe Rylance's theory that he was actually several different writers? "I believe he was one man," he says without hesitation. "Because denying that would mean accepting that a man from a normal background who is perhaps not that well educated, like me, is not capable of achieving great things."

But by the sound of things in the rehearsal room, Flynn is already managing to achieve greatness.

"As You Like It" runs until 5 September at Shakespeare's Globe, London.