Nothing, apparently- which surprised me. Still, it's Bergman's story, not anybody else's. He has nothing to say about Gunnar Bjornstrand either.
Elsewhere (it's a stray quote I picked up from reading the obituaries) he compared von Sydow to a Stradivarius- the best and most responsive instrument he ever had.
Bergman was a star-maker, but none of the great actors he mentored and worked with ever shone as brightly under anyone else's direction. This is even true of Max- who was the one who came closest to breaking out of orbit. No-one else knew how to use him; no-one else was making the kind of movie that displayed him to full advantage. After he and Bergman parted company he got pushed a little to the side- playing doctors, Nazis, kings, priests, assorted evil foreigners.- required to import a little of the mana he'd acquired under Bergman to the lesser films of lesser talents. He was always great (except perhaps as Jesus) but greatest when he was young.
It keeps coming back to The Seventh Seal- the first movie he and Bergman made together- and the most iconic. Bergman calls it "an uneven film that is close to my heart." Von Sydow admitted to always being uneasy about the way he'd delivered his lines. It's as if neither of them dared to believe that this extraordinary thing they had been a part of was as wonderful as it seemed to be. I think this is generally true of artists- that the better the work is the more they sweat the small stuff- and regret the little things that mar its perfection. Bergman could never forget that in the scene of the witch's burning you can make out the lights of an apartment block through the trees; or at least he could. I've looked for them and haven't spotted them yet.
Question: Is The Seventh Seal as wonderful as it seems to be?