English actor Richard Griffiths once famously said of Peter Shaffer’s Equus: “It’s not a whodunnit. It’s a why-dunnit”.

That’s because, right from the outset, we know that a troubled boy, Alan Strang, has committed a horrendous act of violence against six horses – which is why he ends up on the couch of psychiatrist Martin Dysart, himself troubled by a midlife crisis and worse.

The play, presented through Dysart’s eyes, exposes the layers bit by bit until we reach the core of the violence. But at the heart of the drama is Shaffer’s fascination with the ecstatic experience – something not available to most of us.

Consigned to the mundane world, Dysart can only wonder at the religious fervour of young Alan’s encounter with horses, especially his favourite horse, Nugget.

This theme is exactly paralleled in Shaffer’s Amadeus, where moderately-talented Salieri is consumed with envy of the youthful Mozart’s genius, and again in The Royal Hunt of the Sun, where the ageing conquistador Pizarro comes face-to-face with the holy king Atahualpa.

One critic called it “god-hunting”, and there’s more than a touch of that, as I find when I catch up with veteran Canberra actor Arran McKenna, who gets the plum role of Dysart in Anne Somes’s coming production.

“I think it still works,” McKenna says. “It’s one of my favourite plays – I also did it years ago with Free-Rain Theatre at The Courtyard Studio.”

“Shaffer likes playing around with ceremony and high ritual and Anne [Somes] has gone with that, although she’s reimagined the horses – you can’t have big horses tearing up a heritage building.

“Psychiatry has moved on since the 1970s, but the themes of passion and worship are still very relevant. It’s a good subject for theatre, where people are very passionate.”

Some people think it’s Alan’s play, but as he points out, it’s Dysart who constructs the story for the audience.

Jack Shanahan, who plays Alan, is a little older than the character, but as McKenna says: “He plays young very well – he brings a really good brain and sensibility to the role.”

The action begins when Alan commits an act of violence against six horses and Dysart is called in – and yes, there will be warnings about animal cruelty at ACT Hub.

His character, McKenna says, has an academic appreciation of the ancient Greek gods, but when confronted with someone who actually lives those beliefs, he’s shocked  and ultimately envious.

“I’m a person who loves what people are passionate about,” McKenna says, hinting that things don’t look good for Dysart, whose own midlife crisis and strained marriage combine with tricks of the psychiatric trade as he strives to take away the young man’s pain — and with it, his direct contact with the god-like horse figure.

McKenna works by day as a freelance creative graphic recorder, listening at live conferences or meetings and visually capturing key ideas using drawings and text – so he’s a keen observer and listener.

Through modern psychiatry, he suspects, Alan’s pain could perhaps be turned into something productive – but that’s not what the play is about.

You’ll just have to be there to see why-dunnit.

Free-Rain Theatre presents Equus, ACT Hub, Kingston, until November 22.