Peter Weir, director

One morning in early 1973, the TV personality Patricia Lovell knocked on my door. She was thinking of buying the rights for a novel by Joan Lindsay, Picnic at Hanging Rock, a story about the mysterious disappearance of three schoolgirls at an ancient rock formation and she was looking for an up-and-coming director. I had been gripped by the book and was very keen to make it.

In the early 70s, adapting a book was what they did overseas, not in the Australian film industry. Picnic set a precedent, but that didn’t stop us getting the funding we needed, which ended up being A$440,000 (£220,000). Even at that low budget, it was still a dangerous commercial enterprise: a mystery without a solution.

I thought about filming the outdoor scenes in the Blue Mountains in New South Wales, but as soon as I saw the real Hanging Rock in Victoria, there was no doubt. A lot of the visual inspiration came from Australian impressionists of the late 19th century: Frederick McCubbin, Tom Roberts and especially William Ford’s 1875 painting At the Hanging Rock.

To get the soft focus look, cinematographer Russell Boyd, feeling very self-conscious, went to the wedding section of a Sydney department store to buy veils of differing thicknesses to use as lens filters. Some of the dreamlike moments, like the swan representing the “Botticelli angel” schoolgirl Miranda, were scripted; others, like the burst of lorikeets as she enters the picnic grounds, were put together later by the editor, Max Lemon.

A couple of weeks into the shoot Vivien Merchant, who we’d cast as the strict headteacher Mrs Appleyard, was forced to withdraw because of illness. I went back to our original list, and miraculously Rachel Roberts, a British new-wave actor I admired, was free. The wig we’d made for Vivien also fitted her, but she said: “I simply can’t wear it.” It was bad luck in the English theatre to wear other actors’ wigs. Luckily, she had one of her own that she was able to use. She was a consummate performer.

We shot an alternative ending where Mrs Appleyard goes back to the rock to search for the girls. But the last closeup of Rachel, a world of pain on her face, was so powerful I knew it had to be the closing shot – we didn’t even bother to cut together the other ending.

A potential distributor in the US supposedly threw his coffee cup at the screen at the end of the film, saying: “Whodunnit?” He felt he’d wasted a couple of hours. The film was a modest success, but did very good business on college campuses. Picnic was one of those milestones for the Australian film industry as it evolved in the 70s – it was remarked upon around the world.

When I went back to the site years later, they told me people often take pieces of the rock home as a souvenir – but return them, thinking they bring bad luck.

Anne Louise Lambert, played Miranda

In the rough and tumble Australia of the 70s, it was pretty special to have something like Picnic come up. At the end of my teens, I felt I was born to play Miranda. With her free-spiritedness, she felt like an expression of my generation; that kind of flower-people stuff, unaffected by the colonialism of the culture we were raised in.

Peter Weir wasn’t so sure, though. They offered me the part, but reconsidered and gave it to another actor with very little explanation. I was incredibly disappointed. Then, out of the blue, they changed their minds because they thought the other girl was too big – a sign of the times.

All the schoolgirls were lodged together, separately from the rest of the cast, at a Christian women’s guesthouse. It was like stepping back in time, with these sweet little rooms with lots of antimacassars. It was designed to help us bond, but it didn’t happen for me – I was missing my family. With these cliques of young women, there were lots of feelings. Some of them decided that Jane Vallis, who played Marion, another of the disappeared characters, would pair up with Dominic Guard, who played the young Englishman who follows them. Any time I was with him, suddenly there would be this wall of girls in front of me; this instant kind of intervention.

Peter once said Miranda was more of a quality than a character. But the production designer Martin Sharp, who knew Joan Lindsay and the book well, helped me find her. In my room, for example, if I opened a drawer, it was full of things he’d put there that were meaningful for Miranda: Valentine’s cards, handkerchiefs, pressed flowers, photographs in lockets.

I was the only character lucky enough not to have to wear a corset, but clambering about on the rock in those little Victorian boots was brutal. I was still feeling insecure, because there had been something not very wholehearted about my casting. One lunchtime, after a horrible morning filming where I’d had to re-do a scene a number of times, I came down the rock to see a little old woman staggering over some rough ground. I realised it was Joan Lindsay; she came up and threw her arms around me. Right in my ear, very emotionally, she said: “Oh, Miranda, it’s been so long!” That was incredibly validating.

There have been times in my life when I thought the film was a load of fluff, like a shampoo commercial. About 10 years ago, Indigenous groups started objecting to it – which I understand, when the history of the rock is so much greater – and I decided not to talk about the film. But I recognise it’s got this extraordinary life force that keeps it going. I’m a psychotherapist now, and it has this Jungian aspect in the unsolved mystery: the exploration of society’s shadow side, all the murky stuff that is brought up when the girls disappear.

• Picnic at Hanging Rock is rereleased in cinemas on 21 February to celebrate its 50th anniversary