The DVD includes a little excerpt from Bergman's autobiography where he writes about this film. It's a difficult account to get your head around, perhaps because it's a translation, perhaps because Bergman thinks REALLY deeply and seriously about his work (far more than I have done, goes w/o saying). Leaving aside the rather dull debate about whether this constitutes the first part of a trilogy or not (haven't seen the other two, so what do I care?) What was interesting about Bergman's take is that he regretted including the father-son scene at the end. I find this odd because that coda does tie-up a thread left hanging from the conversation between the father and the husband half-way through the film, where the former says he'll reveal what that hope was that saved him from suicide. That scene was so dense I had to watch it twice, and from it the father does emerge as the most interesting character in the piece, much more than Harriet Andersson or Max von Sydow, who Bergman believes delivered the best performances (and sidebar, just how harsh was he about Lars Passgård!)
Gunnar Björnstrand (the father) had recently converted to Catholicism, and Bergman suggests that this made him deliver his lines in bad faith, as if he wasn't truly invested. Which sounds really strange to me because that's (quite explicity!) what the character is all about! The father's bestselling novels flirt with faith and doubt, but the 'truth' is that these themes are tricks and evasions disguising the 'void' of detached unfeeling that is the core of his being. For the purposes of the film's narrative, this manifests as the temptation to mine his daughter's real-life breakdown for themes to supercharge his fiction and achieve that poetic immortality that has so far eluded him. But in that suicide attempt, something did emerge from the void. A love for his son and daughter, of the kind the husband has. The husband cannot imagine leaving his ill wife, his love traps him, he has no freedom. During the trip to Switzerland, similar feelings have been activated for the father as well.
In that final scene, the father rather soppily puts forward the idea that God cannot be found anywhere but in the human capacity to love. Love, hope and God are bound up together. The hope being that the daughter can recover. The method, love. The malady, the lack of love. The side-effect, the God delusion. What is suggested here is that Karin's schizophrenia is caused by her need for an father-figure. Her real father is unreachable, so she imagines another one. In the climactic scene in the attic, she is waiting for God to step into the room. Her father is framed by the doorway, but paralised with shock and unable to come in.
Bergman's notes on Karin's illness are scattershot, suggesting to me that he developed her character very organically, so I'm not sure how much there is to read into the voices she hears or her seduction of her brother. The siblings both demonstrate greater powers of imagination than their father. The husband is perhaps the least spiritually sensitive of the lot, describing himself as a simple man able to face up to life and the real world. He is a doctor and a scientist, and finds it impossible to pray with his wife in her madness. Karin is frigid with him, but flirtatious with her talanted brother, and her yearning for God (the father) is partly sexual. Her husband is a good soul, but he's also patronising (all those diminuitive pet names, my "little child"), and I want to read her illness as a result of her being trapped, creatively and sexually, in an unfulfilling marriage (just as the husband feels trapped, in fact). This territory is definitely covered in Persona, but I'm not sure if it's anything more than background here.
Bergman dedicated this film to his wife (at the time). He describes the way they fell in love through writing letters to each other, opening each other up that way, and then slowly drifting apart as they lost a common language to communicate in. The only way this feeling makes it into the film is the father's desperate attempt to cover up his detachment from his family. And the film, unlike the marriage, ends on a hopeful note, with a relationship being established between the father and son. It IS a soppy ending, it DOES disperse some of the tension of the previous scenes, but its content is crucial to making sense of what came before. Dramatic heft is sacrificed for thematic clarity, and it's clear (from Persona) that Bergman increasingly preferred not to make that trade-off.