“ARE there doors? Do they open? Do they have handles? Are they just doors in the mind?” These are some of the questions mezzo-soprano Karen Cargill put to director Matthew Lenton during the first week of rehearsals for their new production of Duke Bluebeard’s Castle. It’s been a discomfiting spring at Scottish Opera. First we had Pelleas et Melisande, Debussy’s intoxicatingly suggestive and inconclusive opera with its damaged relationships and moral murk. Now Bluebeard.

Bartok was 30 when he wrote his only opera in 1911; it premiered in Budapest seven years later. The score is a many-splendored masterpiece of early modernism, and though it lasts only an hour – Vanishing Point’s co-production with Scottish Opera pairs it with a new work by composer Lliam Paterson called The Eighth Door, a kind of abstract prequel – the dramatic impact of Bluebeard alone is immense.

The libretto by Hungarian socialist playwright Bela Balazs is a brutal take on an already brutal old European fairytale. A young bride wonders why the windows in her husband’s castle are boarded up and what might lurk behind his seven locked doors. She persuades him to open the doors, one by one, and behind the seventh she discovers his previous wives, imprisoned but still alive. Bartok’s musical treatment is stunning: huge orchestra, sumptuously deployed, and just two singers on stage swamped by the forces around and beneath them. Their vocal lines are all about the natural speech rhythms of traditional Hungarian balladry and unadorned, folk-rooted melodic contours. “It’s like a drug,” Cargill describes. “The build of it all. The anticipation of every door. I don’t even mean Door Five” – one of the most majestic moments in all opera, when Judith lets out an ecstatic gasp over a blast of C major in the orchestra and added fortissimo organ chords that glow so deeply you should feel them in your stomach. “No pressure,” Cargill groans.

But back to the danker stuff. Balázs was steeped in French symbolist drama and the influence of Maurice Maeterlinck, whose play forms the libretto for Pelleas et Melisande, looms large. Bluebeard opens with a spoken prologue in the darkest of fairytale traditions: “Where is the stage, Ladies and Gentlemen? Inside or out?” Already our perceptions begin to blur. If much is hidden from view in Pelleas, similarly we never understand why Bluebeard does the things he does. There’s a mystery to his motives that lingers even after all the castle doors have been opened. As with real-life sex abusers, this fairytale Duke is scary because he’s beyond reason. Maybe that mystery is what attracts Judith. Maybe Bluebeard doesn’t understand himself.

“I think,” Cargill ponders, “I think Judith is a woman in pursuit of perfection in a relationship. In pursuit of the deepest possible understanding of another individual.” Productions of the opera often portray Bluebeard as a monster and Judith as a bewildered naif, but the relationship is undoubtedly more complex than that, and the power dynamics between them shift all the time. Why does she follow him into the castle? Why is he so unable to reform his behaviour?

“Neither character can be one-dimensional,” says Cargill. “Judith has got to have power in this. She coaxes him. She wants things from him. It’s important we have two characters who are fully rounded.” She describes the moment when Door Seven is opened and the previous wives are revealed – “they’re still living!” Judith whispers. Here Bluebeard sings a tender song in a simple major key.

“It’s the most open he ever is with her,” Cargill notes. “The music is devastating. She believes him because she loves him, and when we’re in love with someone, maybe we accept things about them that we wouldn’t otherwise. She knows there are rumours about him. Maybe there’s a certain mystique around him, maybe she’s entranced by that. But whenever he addresses her it’s always in a loving way and that tenderness is visceral. There’s something broken about him. Maybe she thinks she can fix it. Maybe she feels she can make him better.”

This is the first time Cargill has played Judith: the role had been discussed for her on various occasions, but she always held off until now. “I knew I wouldn’t have sung it how I wanted to if I’d done it earlier in my career,” she says. This wasn’t a notes thing: Bartok’s orchestra is huge and the singers need to have serious decibels to cut through, but Cargill has earned her stripes as a Wagnerian at the world’s top opera houses. She can do big sounds. It wasn’t a text thing, either – the Hungarian is daunting, she says, but the darkness of her natural Scottish vowels helped with that. Her reluctance was more about getting the character and the colours right. “Ultimately it’s about relating to another individual. I’m not sure I would have had the emotional maturity.”

So what changed? Three things. One, she sang a lot of Wagner – and she does relate Judith to some of the great Wagnerian roles: the goddess Fricka, the valkyrie Waltraute, both capable of almighty vocal squalls when they’re trying to get something they can’t have. Two, she was long overdue a return to Scottish Opera in a role that befits her career trajectory. Her last performance here was way back in 2009 as Rossini’s Italian Girl in Algiers. The company under its current management has a poor track record of celebrating Scottish singers, even those as internationally lauded as Cargill.

Three? “I lived a little,” she says. “And that is essential for Judith. I’m trying to be more calm and more accepting about the way I am as an artist. When you’re a student, you’re obsessed with perfection, then you come to learn that life is not perfect. Going through relationships and births and deaths and breakups – I call on all of that. It’s about getting to the point where I feel I don’t have to be perfect. I just have to tell the story and tell it as honestly as I can.”

Working with Lenton and the Vanishing Point team seems to have helped with that. At the beginning of our interview Cargill refers to herself as an actor rather than a singer – “the amazing thing for me as an actor has been the pursuit of honesty and truth in this production.” Later she jokes that even saying the word out loud caused her a little flurry of self-doubt and identity crisis. “But Matthew has been tremendously enabling. He’s given me permission to think of myself in that way. Yesterday I got a text from Rob [Hayward – the bass playing Bluebeard] saying he has never been so stimulated by another actor on stage. The magic of Vanishing Point is they want to leave things open so we can all make up our own minds. This is not standard operatic stand-and-sing. There will be no Verdi hands here.”

Duke Bluebeard’s Castle and The Eighth Door is at Theatre Royal, Glasgow, tonight until Saturday then the Festival Theatre, Edinburgh, Wednesday April 5 to Saturday April 8.