Anyone who's read Goodnight Mister Tom, the wonderfully evocative and spellbinding story of a young, traumatised evacuee being sent to live with a grumpy old man in the countryside during WW2, will know how integral Sammy is to the story.

Mister Tom’s beloved dog is brought masterfully to life by Elisa de Grey, who throws her heart and soul into conveying Sammy accurately, watching border collies obsessively on YouTube to prepare for the part.

The 39 year-old actress says that while Sammy might be a puppet, he is a vital part of the story, not only as a companion to the isolated and lovely Mister Tom, but as a constant throughout the heart-wrenching tale.

“Sammy is not just there to amuse the children in the audience, or as an extra theatrical tool, but to highlight the solidarity of Mister Tom, especially when the play goes to a darker place, because Sammy doesn’t judge or offer an opinion. He is loyal and uncomplicated, regardless of what is going on in the world.

So where did she start? “I had to learn how border collies move and work, how they differ from other dogs, their characteristics,, their energy, things particular to their breed.

"I take into account Sammy's breathing, eye line, weight, because obviously a puppet is much lighter than a dog, and replicate his energy while capturing his animality, as well as his pitch so that I can vocalise Sammy's sounds.

"It’s a very physical part, the most strenuous I have ever taken on, and the most intense - I basically have to hyperventilate for two hours because a dog breathes twice as fast as we do," she laughs, "so it’s enormously involving while being massively rewarding. "

To improve her stamina, Elisa has a strict regime of yoga, physio, exercise and diet to prepare herself for the role. “I have to be energetic, enthusiastic and present on stage and that takes a lot of concentration. As an actor my work is to be invisible while bringing Sammy alive.

"But I’m also doing what I love, so physical theatre suits me, and everyone in the cast loves Sammy, even though they know he's not real. They are all very attached to him."

So what’s her favourite part? “Sammy’s relationship with Mister Tom, which means that David Troughton and I have developed a special bond because they are never without each other. Sammy is basically an extension of Mister Tom and a counterpart to the war, abuse and loneliness going on around them so we have a very unique and special relationship."

And what of the more bleak aspects of the play? "It’s very tempting to shield children from the harsh realities of life," Elisa says, "but in the long term it doesn’t do them any favours. So the point of Goodnight Mister Tom is ultimately one of salvation and redemption. There is always hope.

"And in these days of technology, we take this to the opposite extreme by asking the audience to participate in the puppetry illusion and therefore become part of the show. And that transition is quite a magical thing."

SEE IT

Goodnight Mister Tom is at the New Theatre, George Steet, Oxford, from Tuesday, March 22, until Saturday, March 26

Atgtickets.com/oxford

0844 8713020