When will it end? Why did I agree? What am I doing here?”

These were the questions that reverberated in the mind of Israeli singer-songwriter Yasmin Levy, one of the world’s most celebrated performers of Judeo-Spanish Ladino music, as every morning she made her way from her temporary home in north London’s Swiss Cottage to the National Theatre.

Once there, gruelling rehearsals would begin. This was Levy’s first theatre production. Director Yael Farber had asked her to be part of Salome, a hugely ambitious take on the myth surrounding Herod’s daughter.

Levy is known for melding Ladino and Flamenco, and has a voice to suit — as fearless as a flamenco dancer in full flow. But she was so “far out of her comfort zone” she was feeling that she had made a huge mistake.

Then came the reviews for the production. Many acknowledged the rare sense of biblical antiquity generated by the piece and its stunning imagery — in the JC, I judged it “utterly authentic but fatally ponderous” — but they were still some of harshest for a National Theatre production in recent times. They came as a shock. Levy is used to acclaim (as is Farber) and has been twice nominated for Radio Three’s World Music awards.

“It was difficult mentally and physically,” she confides when we meet at the National an hour before her warm up. “I stopped my life and had released a new album in Hebrew in Israel and just as it was peaking in Israel I stopped and came over. I’m not saying I regret that.”

Salome is a work whose period (about two thousand years ago) and place (Roman-occupied Jerusalem) is immediately evoked by Levy’s voice.

On stage for nearly the whole of the play’s uninterrupted one hour and forty five minutes, she plays a mystical “woman of song” who circles the action as her music interweaves with that of Syrian opera singer Lubana Al Quntar, who performs traditional Syrian songs.

“It was hard for me to stay with another singer,” Levy admits. After all, she normally performs for an audience who has paid to see only her, not a whole company of performers. “But with time and with modesty… I’m not a diva. I serve the play.”

Some of the music sung by Levy is not ancient but was composed by Levy “on the spot” during rehearsals.

“I also brought the music of my home to the stage. Some of the songs are Sephardic prayers of my childhood, which I listened to in synagogue,” says the Jerusalem-born artist who perhaps more than anyone involved in the production personifies the city in which Salome is set. She lives there with her husband and two young children opposite the Mount of Olives.

But although her first play has been a bruising experience, it has, she says, strengthened her as an artist.

“The cast are defiant. We’re proud of this play. And we give more than before. It has made me braver. And it will makes me more fearless for my next album.”

‘Salome’ is on at the National Theatre until July 17. It will be screened at cinemas across the UK as part of the NT Live series on June 22.