In a white-walled rehearsal studio in Waterloo, Kathryn Hunter – muse to Peter Brook and Mike Leigh, artistic associate of the RSC, one of the great actresses of our age ... is sprawling, starfish-like, on a small purple yoga mat. Slowly and silently, she finds her feet and, semi-standing, begins to lope deliberately across the room, knuckles brushing the floor, arms swinging. Eventually she stops, squats on her haunches and hugs herself contentedly for a while before shuffling over to pick some imaginary bugs out of the hair of a crew member, munching on her find as a satisfied moue settles on her mischievous face. Apart from some ambient jungle noises on the stereo and occasional, gentle instructions from the movement coach, you could hear a pin drop in the spellbound studio. Hunter is becoming a monkey in front of my eyes and it is quite extraordinary to watch.
But why? The answer lies, where else, with the master of absurdly dislocating experiences, Franz Kafka. Hunter is currently deep in rehearsals for Kafka's Monkey, an adaptation of the Czech author's short story, "A Report to an Academy", in which a monkey describes his transformation into a man in front of an audience of curious experts. In a one-ape show, "Red Peter" recounts the story of his capture by sailors and his dawning realisation that his only means of escape is to become human. To this end, he learns to spit, smoke, drink rum and speak like his captors, an incredible metamorphosis which eventually leads to a lucrative career as a music-hall entertainer – and a prestigious invitation to address the mysterious academy.
At the Young Vic, then, Hunter will play a monkey who is, in turn, playing a man. Later in rehearsals, she dons Peter's vaudevillian bowler hat, tailcoat and white gloves to practise the lecture. The effect is startling. Here is a tiny, comical figure, clutching a whisky flask and silver-topped cane, confidently delivering a report, dark eyes glittering all the while. Aside from the extraordinary nature of his tale, there's nothing too obviously odd about it. But then you realise there's something not quite human about him – his feet are a little too turned in as he walks, his jaw occasionally slackens and pushes itself out, his fingers are a little awkward as they unscrew the cap of his hip flask. In its disorientating, almost grotesque, blend of human and simian, Hunter's performance is the word Kafkaesque made flesh.
"We're checking the dosage of ape to human at the moment, and playing with it," explains Ilan Reichel, director of movement. "Sometimes it's 5 per cent ape to 95 per cent human. Sometimes it's the other way round." As roles go, it's immensely challenging. But if anyone is up to the job, it's Hunter. The 51-year-old actress, a diminutive, lithe figure with deeply expressive eyes and a stage-stealing physicality, has made her name by being unusually adaptable. She has played both King Lear and Richard III, a nine-year-old autistic cancer sufferer (in Spoonface Steinberg) and, most recently, a couple of ancient crones in Samuel Beckett's Fragments and has worked for many years with the physical theatre troupe Complicite. "It's the best casting I've ever done," beams the director Walter Meierjohann. "She's so perfect. She has an ambiguity within and an incredible range so she can play young to old, female to male. This creature is also somewhere in between. We're always asking, 'What is it?'"
As the monkey-turned-man, Hunter must balance on a knife-edge between the two, occasionally allowing her simian impulses to erupt before her human side suppresses them. First, though, Hunter has to find her inner ape via a rigorous training schedule and up to three hours a day spent in animal movement exercises. These usually begin with a half-hour physical "monkey warm-up" with Reichel, a spry, white-haired animal studies teacher in MBT trainers. The head of movement at Rada for 21 years, where he taught students to embody every living organism, from single-celled amoeba to elephants, he has developed a specific process, using observation, dance and improvisation, to capture the essence of a creature.
In the studio, the floor is strewn with pictures of chimpanzees, the walls lined with anatomical sketches and the skeletons of apes. To start with, Reichel and Hunter analysed the bone structure of the chimpanzee, discovering its lower centre of gravity and exploring its use of its forearms to push itself away from the ground. After that, they move on to improvisation. Every little action of Hunter's – from picking up a glass to climbing on to a chair to brushing hair away from her face – is imbued with a newly measured and thoughtful quality to distance it from normal human reflexes.
This way of working is not entirely new to Hunter. In Complicite's celebrated production of The Visit, as Clara she crept around the stage "like a spider" – a performance which won her an Olivier Award. "It's not about galumphing around but about getting inside the animal's being," she says. "It's also being precise about things like bone structure and dynamics – not just imitating and pulling silly faces."
As with his most well-known work, Metamorphosis, in which the hero Gregor Samsa wakes one day to find he has become a hideous bug, Kafka's short story about a man-monkey is ripe for reinterpretation. Over the years, it has been read, variously, as a diatribe against Jewish assimilation into Western culture (it was first published in 1917 in a Zionist magazine), a debunking of the Darwinian theory of evolution and the supremacy of humans (in the tale, the sailors are depicted with almost bestial qualities while Peter assumes a superior tone), a meditation on freedom, an empathetic rallying call for animal rights, a satire on colonialism and a deconstruction of the Freudian ego.
Meierjohann has settled on the all-encompassing theme of identity and self-alienation, which questions Peter's motives in shape-shifting. "How much does it cost to adapt to an identity?" he asks. "And at what cost?" The director has assembled a team with a specifically international make-up to further explore the idea of alienation within a culture and the tricky nature of assimilation. Hunter is a Greek-American, now living in England while the rest of the company is drawn from Germany, Macedonia, Israel and Denmark. "We all share the experience of what it feels like to have to adapt," explains Meierjohann. "Sometimes you do feel like a monkey when you don't know how to queue or how to be polite. Not knowing that if someone stands on my foot, I should apologise, that kind of thing. It's about otherness."
Last week, the cast and crew went on a jaunt to Whipsnade Zoo to visit its monkey enclosure. Hunter was particularly struck by the human qualities of the chimpanzees she saw there. "When you look into their eyes, the feeling of 'There's my relative, there's my distant cousin' is something that strikes most people. There's something very recognisable there."
"It's the way they look at you," adds Meierjohann. "One of the zoo-keepers said to me that he always wonders what they must think of us looking at them, which is exactly the feeling we're trying to get across on stage. To hold a mirror up to human existence."
'Kafka's Monkey', Young Vic, London SE1 (020-7922 2922), 14 March to 9 April