They are kept busy with quick changes - many on stage - of costume, character, accent, even sex. And occasionally someone must play more than one character at a time or even an inanimate object.

Nathan Page - the fourth actor, who plays only one role, the 139th - says of the South Australia Theatre Company production, "It's Hitchcock on steroids, really."

This production is a remounting of the company's successful 2016 production with the same cast as before. Original director Jon Halpin could not return and the 2019 production has been mounted by associate director Corey McMahon.

The 39 Steps was adapted from the 1935 Alfred Hitchcock film based on John Buchan's novel - it tells the story of Richard Hannay (Page), a bored, ordinary man in London who goes to the theatre one night and suddenly finds himself swept up in murder, intrigue and mystery.

Page says that, "As part of the rehearsals, at the start we read the book and watched the film to familiarise ourselves" but adds the play is "a completely different kettle of fish, really".

It tells the same story as the film - with many of the same situations and characters - but in a more comic way.

McMahon says, "Patrick Barlow's adaptation is clever, witty, and at times downright ridiculous. He skillfully and faithfully preserves the essential ingredients from John Buchan's novel and Hitchcock's film adaptation and then up-ends it all as our company of actors desperately try to pull off presenting one of the great, epic spy-thrillers on stage, with hilarious results."

The 39 Steps starts off in London in the 1930s. While Hannay is watching "Mr Memory" show off his powers of recall, a gunshot causes chaos and he ends up taking a frightened woman, Annabelle Schmidt, back to his flat.

Annabelle tells him she is a spy being pursued by assassins who want to kill her. She says she has uncovered a plot to steal vital British military secrets and mentions a mysterious something called "The 39 Steps".

The next morning, Hannay wakes up to find Annabella has been murdered and is clutching a map of Scotland.

Knowing he will be a suspect in the crime and wanting to get to the bottom of Annabella's story, he sets off on an adventure that will see him travelling by various means - on foot, by train - to different parts of the country.

Hannay, Page says, is being pursued by the police as well as the villains who killed Annabella and must rely on his wits to evade arrest or worse while trying to find out what's going on and foil the plot.

He just might find time to fall in love along the way. But that, like the rest of his self-appointed mission, won't be easily accomplished.

The company had only two weeks to rehearse the new mounting of The 39 Steps.

Having already done the play together made it easier to cope again with the challenges of the show's rapid pace and quick changes of scenes, settings and characters.

Page says McMahon helped them bring out "a little more nuance" in the characters and situations which helped keep up their interest and made the new production feel less like a rehash.

Nathan Page is a longtime stage and screen actor well known for his role as Detective Inspector Jack Robinson in Miss Fisher's Murder Mysteries.

He has recently been in Morocco - which stood in for Palestine - filming the upcoming movie spin-off from the TV show, Miss Fisher & The Crypt of Tears with Essie Davis returning as Phryne Fisher.

"It was amazing," he says.

"I'm look forward to going back."