Pernilla August: Bergman, Jesus, Darth Vader - I have mothered them all

In one respect, Ingmar Bergman is rather like Status Quo: when he says goodbye, it's usually au revoir. In 1982, weighted with bouquets for Fanny and Alexander, he announced his retirement from the cinema. Since then, he has written three film scripts – entrusting the camera to his lieutenants Bille August and Liv Ullmann, and his son Daniel – and directed nine TV movies – the most recent of which, Sarabande (2003) has been snapped up by Sony for worldwide theatrical distribution. In 1995, he bade his first final farewell to the stage, but only managed to keep away for three months. Adventures with Strindberg, Schiller and Euripides have followed. Now, as Bergman approaches his 85th birthday, his latest work for the theatre – a production of Ibsen's Ghosts – visits the Barbican on the way to New York. According to Pernilla August – its Mrs Alving, and a Bergman collaborator since 1982 – it is the director's stage swansong. Goodbye, farewell, auf wiedersehen, adieu. And this time he actually means it.

"This is the last one," she sighs. And she struggles to find a metaphor to express the air of finality that she can sense in rehearsals. "In this production, I can hear echoes of other plays of his that I've been in. I can feel winds blowing from all the plays and movies that he directed. I don't know whether the audience will feel that, but it seems very clear in the rehearsal room. This is the last thing he's doing. It's like he's gathering everything together in one play. Even in the set, you can see him going through this process. We have the same sofa that we used in A Doll's House, the same statue we had in The Ghost Sonata. He's saying goodbye. And in a very beautiful way." Pernilla August – former spouse of Bille, mother of Asta, Agnes and Alba, pillar of the Swedish stage – has developed an interesting specialism.

Directors turn to her when they want their mummy. Bergman cast her as his mother in The Best Intentions (1991), Kevin Connor signed her to play Our Lady in Mary, Mother of God (1999), and for the current Star Wars trilogy, George Lucas has enlisted her to give birth to Anakin Skywalker. "Perhaps this says more about them than it does about me," she reflects. (And also suggests, she volunteers, that Bergman, Jesus and Darth Vader are half-brothers.)

Bergman's shadow has loomed large over August's career. She decided to be an actress at the age of 15, during a screening of Cries and Whispers (1972). Gazing upon Kari Sylwan's performance as the maidservant of the dying Harriet Andersson, she wondered whether she might ever get the chance to fill such a role. Ten years later, she was on the set of Fanny and Alexander (1982), playing the nanny in Bergman's romanticised portrait of his childhood. (She was dizzy with nerves on the first day of filming, and hugely relieved that Bergman chose to inaugurate the shoot with a scene of pillow-fighting.) Playing Bergman's mother in The Best Intentions (1991) was a further sign of the Master's approval, and in the theatre, she has become his most trusted collaborator. She has been his Ophelia, his Mary Stuart, his Nora, his Hermione. Bergman, she says, has even altered the timbre of her voice – her self-deprecating impersonation of herself before he began working on her larynx is like Kermit the Frog doing Joe Pasquale.

Unlike many of Bergman's other muses – Bibi Anderson and Liv Ullmann, for example – Bergman's relationship with August remains platonic. She has never been part of his household, never stayed with him in his retreat on the island of Faro. "We have no private relationship," she says. "We speak to each other on the phone maybe once a year. All the children have to be quiet and leave the room for one hour! But otherwise I've been working for him for 23 years, so he knows me better than anyone else. He's known me since I was a young woman, and now I'm a middle-aged woman.

"In terms of my acting, he's helped me make the transition from one to the other." The next stage of her career, however, will be conducted without Bergman's guidance. She speaks of it as if she is in the process of negotiating the amicable dissolution of a long marriage. "It's important for me to go on, to work with other people. And I know he wants that too. There's so much love between us, and it's really so deep in me. One day I will understand how much he has meant to me. I need more distance to understand that."

His valediction, it seems, is even legible in his rehearsal-room manner: "Sometimes he can be very prescriptive in the way he directs. He'll tell you how to walk, when to turn. When I did Mary Stuart for him, it was like a ballet. Two steps that way, three steps another. This time he is more relaxed. He listens to my impulses. He has set me free."

'Ghosts': Barbican Theatre, London EC2 (0845 120 7500) Thursday to 4 May