Up and coming British actress Catherine Steadman, who has recently appeared in TV’s Downton Abbey and in Richard Curtis’ film About Time, stars in new West End play Oppenheimer about a group of Berkeley masterminds pioneering the use of the first atomic bomb.

“The wonderful thing about Tom Morton-Smith’s characters is that their motives are fundamentally understandable and human. Whilst you may not agree with what the characters do, you do understand why they are doing it,” enthuses British actress Catherine Steadman who stars in Morton-Smith’s new play Oppenheimer which made its West End debut in March.

Beginning in 1939, the story takes place against the backdrop of growing fascism in Europe where the discovery of atomic fission by two German chemists threatens to change the world as we know it. Thousands of miles away across the Atlantic ocean, theoretical physicists in Berkeley, California recognise the horrendous potential of this new science: a weapon that draws its power from the very building blocks of the universe. J. Robert Oppenheimer finds himself at the centre of what could potentially be the most significant scientific undertaking in all of human history.

Struggling to cast off his radical past and thrust into a position of power and authority, the ambitious and charismatic Oppenheimer races to win the “battle of the laboratories” and create a weapon so devastating that, with the detonation of a single device, it would bring about an end not just to the Second World War but to all war.

On Tom Morton-Smith’s writing: “The wonderful thing about Tom Morton-Smith’s characters is that their motives are fundamentally understandable and human. Whilst you may not agree with what the characters do, you do understand why they are doing it.”

Steadman plays psychiatrist Jean Tatlock, who the actress describes as being “an amazing woman; an idealistic socialist and the great love of Oppenheimer’s life”. Similarities with The Imitation Game, both in story’s World War II setting and the romantic entanglement between two great academic minds, are unavoidable but hugely relevant. Indeed, while the real-life relationship between Tatlock and Oppenheimer (who is perhaps better known as the “father of the atomic bomb”) forms a central part of Morton-Smith’s play, it’s Tatlock’s courageous endeavour in a “man’s world” that makes her story as significant as Keira Knightley’s Joan Clarke in Morten Tyldum’s film.

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Steadman says the challenge is being respectful to the person when dramatising their real life. “There is always an innate responsibility involved in playing a real person because you want to do justice to who they were and their situation. So that was a challenge – but during the rehearsal period we read a lot of material around the characters, and luckily I managed to find a biography by authors that had had access to some of Jean’s private letters and photographs which really helped to get an insight into her life and what motivated her to do what she did.”

The actress, who was born and raised in London and attended the Oxford School of Drama, auditioned for the part of Tatlock in the summer of 2014 while she was filming popular television series Downton Abbey. Certainly, period drama is a genre Steadman is familiar with having made her screen debut as Julia Bertram in ITV’s adaptation of Mansfield Park before going on to play Mable Lane Fox in season 5 of the network’s multi award winning series about the aristocratic Crawley family.

On her character: “There is always an innate responsibility involved in playing a real person because you want to do justice to who they were and their situation. So that was a challenge – but during the rehearsal period we read a lot of material around the characters, and luckily I managed to find a biography by authors that had had access to some of Jean’s private letters and photographs which really helped to get an insight into her life and what motivated her to do what she did.”

Having met with Morton-Smith and the play’s director Angus Jackson to read scenes, Steadman noted how fascinating it was to immerse herself in the lives of such brilliant minds. Epic by nature, the characters involved each brought something unique to the Manhattan project, shaping the end of World War II and the future of modern warfare and international relations. Yet within its themes of science and technological discovery, the story has a very human centre.

Steadman describes Tatlock as a “firebrand but she suffered what might now be diagnosed as manic depression.” It was her ambitious passion for discovery that helped her battle both personal and institutional demons to successfully support Oppenheimer’s endeavours.

Indeed, there was even time for some fun. “As well as being groundbreaking scientists they were also all in their late twenties or early thirties and loved to drink and throw wild parties. The stories of the things that happened at Los Alamos are absolutely fascinating and hilarious.”

It’s the roots of human existence that powers the play. “Tom [Morton-Smith] manages to make the science of the play both understandable to laymen whilst retaining the excitement of discovery that the young scientists would have felt at the time. It was such an exciting time.”

Transferring from a successful run at the Swan Theatre in Stratford-upon-Avon, Oppenheimer began previewing in late March and will run until May 23rd at the Vaudeville in the West End. Commissioned by the Royal Shakespeare Company, the epic drama takes us into the heart of the Manhattan project, revealing the personal cost of making history.

The Royal Shakespeare Company’s artistic director Gregory Doran expressed his joy that London audiences were going to get the chance to see Morton-Smith’s play. “The epic nature of the subject and the broad sweep of the narrative is something we have always encouraged in our commissioned new work. This is a direct legacy of Shakespeare, who, of course, was once a ‘new writer’ himself.

“Angus Jackson’s original production opened in the Swan Theatre which is not only a showcase for Shakespeare’s contemporaries, but also for today’s writers who have been inspired by him. Tom took up our challenge to think big, and has tackled a complex subject and some extraordinary personalities, and has woven a compelling narrative around them.”

While Steadman believes audiences will experience a “fun and fast-paced show” they’ll take much more away from the play. “They’ll have learnt something and probably want to Google-search a lot of the characters and stories and find out more.”

On the joys of stage acting: “Stage is much more audience responsive than TV/film as you get a real sense of the audience going with you on the journey of the play.”

While the actress will be concentrating on Oppenheimer for the next two months, she hopes to continue mixing stage with television and film roles. Many will recognise her from Downton Abbey but she has appeared in several television programmes including playing Nurse Angela Wilson in ITV’s 60s drama Breathless. She’s also featured in The Inbetweeners, Holby City, Law & Order: UK, Missing, Lewis and Fresh Meat. Last year, she played Kate in comedy series Trying Again and has guest starred in Quirke. Her film roles have included Salmon Fishing in the Yemen and Richard Curtis’ About Time.

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“It’s hard to chose between them because I enjoy them both,” she said when asked whether she favoured stage or screen. “They’re so different. It’s just great to get the chance to go between the two to be honest.”

But the stage can be quite liberating, she tells me. Is stage more of a challenge without the benefit of getting a second “take”, I asked. “In a way you do get another “take” each time you do the show as your performance changes from night to night,” she replied.

The biggest difference is in vocalising the character, says Steadman. “For film and TV, actors are mic’d up so you just need to talk at a normal conversational level whereas for theatre you need to reach the back of the upper circle of an auditorium.

“Another key difference is that film/TV is shot out of sequence and you would only ever be expected to film a couple of scenes a day as you would need to shoot each scene from several angles. With stage you start at the beginning and run right through.”

There’s also the added excitement for an actor to have his or her audience directly in front of them. “Stage is much more audience responsive than TV/film as you get a real sense of the audience going with you on the journey of the play.”

Catherine Steadman can be seen alongside John Heffernan in Oppenheimer at the Vaudeville Theatre in London until May 23rd.

Written by Daniel Stephens