Sheila Atim would not call it a shrine, exactly, but in her London flat, there is a mantelpiece on which she has placed a few commemorative bits and bobs. There’s an Olivier award here, an MBE there. “My friend did say it looked like a shrine,” she admits, laughing. “But it’s not because I want people to walk into the flat and be blinded by the lights of Sheila’s achievements. These things are just nice! They’re little milestones. I look at them and go, whoa. What?!”

There’s a lovely, knockabout energy to Atim, balanced by a deeply serious side. More than almost anyone I’ve interviewed, she really, properly, thoroughly considers every angle of every question she is asked, as if she’s been set a puzzle to solve. Perhaps that’s why casting agents spot a kind of otherworldliness in her. “A little bit, not bang in the middle of the mainstream, just slightly to the left,” as she puts it. Atim has been a powerful presence in the theatre world: she won the Olivier in 2018 for her role in the Bob Dylan-inspired musical Girl From The North Country, and has done almost every interesting, just-slightly-to-the-left production of Shakespeare going. Then, in December, she was awarded the MBE – at the age of 29. Now she’s moving on to the small screen with a BBC primetime debut in which she’ll be playing a witch. In The Pale Horse, a list of names is found on the body of a woman who has been murdered; a man who finds his own name there becomes obsessed with a trio of spooky women in a leafy suburban village. It’s an Agatha Christie adaptation, which means Atim can’t tell me anything else about it. “It’s quite hard, isn’t it, with a whodunnit? But I am playing a witch. That much I can tell you.”

It will be her first time as a witch, other than a stab at Macbeth in drama classes. “But I’ve played lots of ethereal characters. Or characters who have a supernatural element, or are ambiguous as to whether they’re real or incorporeal.” Why does she think she gets those parts? In person, Atim is wiry, eager and sharp, and lacks the wafty vagueness that “ethereal” conjures. We meet on a freezing cold night and sit in a photographic studio in an old railway arch, huddled around a radiator, wrapped in coats and scarves. Every now and then a train rumbles overhead, adding a touch of the theatrical whenever the conversation turns introspective.

“I tend to be quite a pensive person, I guess,” Atim says. “I ruminate a lot and can be quite quiet when you first meet me, so I think that makes people think, ooh, something’s going on there. Ooh, mysterious.” While there are plenty of actors who tend towards the insular, most are show‑offs; it’s the nature of the job. Does she not jazz hands it up a bit at auditions? “Oh no,” she says, mock appalled. “I just can’t. I’ve always been very bad at networky events, at press nights and things. Everyone is buzzing with this meerkat energy” – she does a convincing impression of a meerkat, head bobbing up and down, left to right – “looking to see if there’s someone more famous over there.” As an only child, she’s happy being on her own, whether that’s at home under a blanket, or wandering through a party. Is she a shy person? “I definitely have a shy streak. I’m a very cautious person. I’ve realised that, as I’ve got older.”

With Arinze Kene in Girl From The North Country. Photograph: Tristram Kenton

This caution has served her well at times. At the start of 2019, Atim was announced as one of the stars of Bloodmoon, the Game Of Thrones prequel written by Jane Goldman and starring Naomi Watts. That could have been life-changing, and she had to prepare herself accordingly. They shot a pilot, but it was revealed later in the year that it hadn’t gone ahead (another prequel, House Of The Dragon, written by Rampage writer Ryan Condal, was picked up instead). She seems sanguine about it. “When I found out, I had work coming up, so that helped soften the blow. As much as I wanted it to happen, I was also prepared for it not to; it didn’t completely devastate me. You have to be ready for that kind of stuff.”

Even without Game Of Thrones, Atim is about to become infinitely more recognisable. After The Pale Horse, she’ll be seen in the Amazon adaptation of Colson Whitehead’s bestseller The Underground Railroad, directed by Moonlight’s Barry Jenkins. She plays Mabel, mother of the protagonist, Cora, a slave who escapes from a Georgian plantation and heads north. “It was genuinely the best thing ever,” she says. She was a fan of Jenkins’ work, and he’d seen her in Othello at the Globe in 2018, playing Emilia to Mark Rylance’s Iago (more than one critic observed that she was more than a match for him). “Obviously Underground Railroad is set in slavery times, and you need somebody at the helm who is on top of their game when it comes to that subject matter. [Jenkins] just looked after everyone, and everyone felt safe and happy and excited to be there, despite the kind of story we were telling.”

Does this mean she’s making the big move to the US? Sort of, she says. She got a load of emails just today about various projects. “Some are in America and some are here, some are theatre, some are film, some are TV. I’ve always wanted to keep all those options open.”

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Atim was born in Uganda and grew up in Rainham, east London. She moved to the UK as a baby with her mother, who is a managing commissioner for the NHS. She was going to be called Vivienne, she says, but her mum changed her mind and went for Sheila. “I don’t know why. But I do know [about] my surname. I’m from the Acholi tribe, in northern Uganda, and you don’t necessarily get given a family surname. Your surname has a meaning, and it’s often related to the circumstances of your birth. My mum went with a literal one. I was born in Kampala, but our family’s from the north, so Atim means ‘born away from home’.”

With Rita Tushingham (left) and Kathy Keira Clarke in The Pale Horse. Photograph: BBC

At school, she acted, sang and learned multiple musical instruments, but planned to be a doctor. She applied to medical school, but when she didn’t get a place, one of her teachers told her he thought it was a sign. “It was like a bubble popped over my head. I thought, ahh, I’m gonna be a performer.” She decided to be a singer, but still did a degree in biomedical sciences, because she loved the subject so much. “I’ve always written soul music, but I didn’t really know what my plan was. I liked to sing and accompany myself on the piano, and I’d loved Alicia Keys since I was a kid. Later on, I discovered Nina Simone.” While she was studying, she went to open-mic nights and tried to learn about the music industry. “I had this perception that I needed to write an EP, get four tracks down, work with a producer, get a rapper to do 16 bars,” she laughs. “Now I look back, I always felt like something wasn’t quite whole about that approach. People would say, ‘When’s your music coming out?’ And I’d be like, ‘Ah.’”

She took a few music classes at WAC performing arts school in London, and ended up doing some drama there, too. Her path was set, though she didn’t yet know it. “When the acting took off, I was very resistant to the idea,” she says, smiling. Her drama teacher kept asking if she was going to drama school. “I said, ‘What are you talking about? No, come on. I’m gonna sing a couple of songs, get the rapper to do the 16 bars.’ But then everything started to expand.”

Her first big stage role was in 2016, in Les Blancs at the National, though it was nonspeaking. “I just walked in slow motion! That got people’s attention, but, because I didn’t speak, I think I remained this, ‘Is she even real? Does she even exist?’ figure.” Later that year, she appeared in Phyllida Lloyd’s groundbreaking all-female Shakespeare Trilogy – Julius Caesar, Henry IV and The Tempest – set in a women’s prison. Then came Girl From The North Country and her first proper TV role, playing a Georgian prostitute called Limehouse Nell in the brilliantly scabrous Harlots. “It was cool, because it was a period drama. I thought, that’s probably the last genre I’m going to get into any time soon, and then here I was. But also I was performing Girl From The North Country at the same time, so that was a super-intense week.” Swinging between the two can’t have been easy. “That week was pretty dark, she laughs. “I was in Watford at 5am every day, filming, and then doing the show in the evening. Saturday came and I was just backstage, flat on my back, and somebody had to prod me with a stick to go on and say my lines.”

Girl From The North Country was a gamechanger. Atim played the pregnant, unmarried teenager Marianne, and her haunting rendition of Dylan’s Tight Connection To My Heart (you can find it on YouTube) regularly moved audiences to tears. In fact, she had to watch herself singing recently because her agent wanted to put a clip of it in her showreel. “And I was really moved,” she says. You made yourself cry? She cracks up. “Yeah, I cried at myself and sat in front of my shrine. Popped a little screenshot of it on the shrine and poured some libations.” Seriously, she says, it’s more that people keep telling her how much it’s meant to them. “When I was shooting the Game of Thrones prequel in Belfast, one of the bar staff at the hotel said, ‘I never saw the show, but me and my mum love Bob Dylan and we put you on YouTube.’”

With Jade Anouka in Henry IV. Photograph: Helen Maybanks

When Atim picked up the Olivier for best supporting actress, she looked genuinely surprised. In her speech, she said: “I really hope there will be more women who look like me accepting awards.” Does she feel optimistic? “Do you know what? I really do,” she says. She deleted her social media at the start of this year, and has been enjoying a bit of a digital detox, so she hasn’t tuned in to much of the recent #oscarssowhite or #baftassowhite controversy. “It makes people feel disappointed, and I get it,” she says. “But I’m hopeful because I’m privy to the conversations: I’m hearing people talk about their experiences, talking about what they want to change, what they want to make. There’s a group called Blacktress run by [Harry Potter and The Cursed Child actor] Cherrelle Skeete, which brings [BME] women together. Just knowing that exists, irrespective of what the nominations list is for some awards ceremony, fills me with great hope.”

Last year, Atim wrote her first play, Anguis, which debuted at the Edinburgh festival. “It was a massive, massive punt,” she says. “I didn’t think I’d ever write a play, ever.” She doubted that she could pull something out of her own imagination. “I thought, if a script exists, I can help as an actor – but I couldn’t just create something out of thin air: that’s insane. And then I had all these ideas I didn’t think I’d have.” She was interested in the myths around Cleopatra, whose achievements in science are rarely remembered, and made her into a character. Another character is a doctor, as Atim might have been. Once she got going, she found it hard to stop. “The director said, this is really interesting, but there’s so much of it. I was, like, I know. If you’d read previous drafts, it was just my mind going a mile a minute.”

She’s currently redrafting Anguis, in the hope of putting it on elsewhere, and has a couple of other projects on the go. She doesn’t want to jinx them by talking about them, though she does say they bring music and theatre together in the way she’d always hoped she might. Is it the four-track EP with a rapper, at last? “OK, you got me. Finally! After six years, I’ve got round to it. It’ll be a fire EP and you’ll find me outside Camden station.”

For a long time, Atim says, she felt as if she was getting away with something – as if, by being on stage at all, she’d somehow slipped through the net. “It was only when I picked up my MBE in December that I really had a moment of, oh, you’re probably meant to be here. For a long time, I had this rumbling imposter syndrome underneath.” On cue, a train goes above us, and the whole room rumbles. “Ominous,” she says, though it is anything but.

• The Pale Horse begins on BBC One on 9 February.

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