Just over 20 years ago, I was standing in a car park in France, shooting a film for the BBC called A Foreign Field. It was day two and, as I stood beside the camera, a tall woman walked purposefully towards me. I remember exactly what she was wearing, partly because it was cold and partly because it wasn't very much: a low V-neck brown cardigan with just a bra underneath, a pair of old blue jeans, and sandals. She was just under 70 years old at the time, but somehow age did not seem important. Lauren Bacall had come to say good morning to her director.

The film was a simple story of two veterans of the second world war returning to the beaches of Normandy, where they had fought. They were played by Alec Guinness and Leo McKern. On the way, they meet up with a Frenchwoman played by Jeanne Moreau, and a mysterious American called Lisa played by Lauren Bacall. Bacall was on set for a makeup test and final costume fitting, but the designer had come to me in tears the night before to say he could not find a single article of clothing Miss Bacall would agree to wear. I looked at the selection: the film had a tiny budget, and all the clothes were from BBC stock. They hung drably on the rail.

I said not to worry. I was feeling confident – the day before, I had presented Moreau with a single green dress as my choice for her. "Come upstairs and let me try it on," she said. She changed in her hotel bathroom and emerged struggling with the zip. "I love it," she said, "and what's so brilliant is that it doesn't fit me at all!" She then produced a pair of bright red shoes and said: "What about these? I wore them in Jules et Jim. They would work, no?'"

Bacall leaned in for her good morning kiss, which I recognised as a ritual. I said, "Good morning" and added: "You realise you've just chosen your costume." We did not at that point know each other very well, and she wasn't sure if I was joking, as her intention was to go to Paris and shop for a costume. "No one would choose what you're wearing," I said. "Jeans, cardigan, no blouse, bra-strap showing, bare neck. Lisa is depressed: she is someone who just doesn't care, who has no interest in how she looks. It's perfect."

She hesitated infinitesimally, then grinned and said: "Got it." This was no small decision because the events of the film covered a single day, so the cardigan and bra would be worn nearly every day of the shoot – although after a week she was getting cold, so I wrote a quick little scene where Alec Guinness lends her a raincoat.

I had chosen a tiny guesthouse in a small Normandy village for the cast to stay in – we couldn't afford a grand hotel, so I needed somewhere eccentric and charming. Bacall and Moreau arrived the same night and wandered through the reception areas picking out pieces of furniture and pictures they liked and having them sent up to their rooms by the open-mouthed and adoring French staff. Together, they built themselves nests and were very happy.

I sometimes wish I had filmed our dinners at the guesthouse's big round table. Bacall and Moreau had been world-famous since their teens and they were both very political: there wasn't a president or major politician they did not know; nor a poet, nor a writer, nor an actor – and it was clear that most of the people they mentioned had been in love with them in one way or another.

It was talk, not gossip. Bacall was a Democrat (anti-Republican, as she described it), and politics rather than film was her passion. She had campaigned for Democratic hopefuls Adlai Stevenson and Robert Kennedy, she knew the Reagans, and she rolled her eyes at the mention of the then incumbent, George Bush senior. But it wasn't just politicians. There were writers (Ernest Hemingway and Graham Greene) and directors (Howard Hawks, John Huston, Sydney Lumet, Robert Altman and of course Bogart, who she referred as "Bogie" as if he was in the next room). It is hard to do justice to what a formidable presence she was, yet there was also a girlishness about her and an easy, companionable intimacy.

Part of a director's job is to find the right way to communicate with each actor; almost everyone has a different route and needs a different kind of language. Bacall appeared to have no map, no feel for a direction, which seemed surprising. She would ask quite openly: "What should I do? What do you want?" As she said this, it was impossible not to see, flickering in front of her, the astonishing cinematic images and decisions she had made in her career – the whistle, the cigarette, the kiss – and yet sometimes she behaved as if it was her first ever day on a set. It was this openness, perhaps, that made her effect on camera so breathtaking.

One other costume was required, for a final graveside scene. She produced her own simple black Armani dress on which we immediately agreed, job done. That night, I got a call from the designer. "She wants a Gucci handbag to go with the dress," he said. "Give it to her," I replied. "I can't," he said. "It costs £800!" So I went to the producer and said: "She's wearing entirely her own clothes in this film. We haven't even had to hire a pair of socks. Let her have the handbag." He agreed.

Later, I said to Bacall: "You've got your handbag, but please leave it in the car when you arrive at the graveyard. Don't carry it in the scene." She raised an eyebrow as usual. "You're going to cry," I explained. "If you're holding a bag, the scene will become about what you do with the bag."

"Got it," she replied with a grin.

The handbag was never seen again. I assume it made its way back to New York's Dakota building, where Bacall lived. So the BBC forked out £800 of licence-payers' money for a handbag that never appears in the film. It was worth every penny.

In her own words

Stardom isn't a profession, it's an accident.

If there was one thing I had never been, it was mysterious, and if there was one thing I had never done, it was not talk.

Looking at yourself in a mirror isn't exactly a study of life.

I used to tremble from nerves so badly that the only way I could hold my head steady was to lower my chin practically to my chest and look up at Bogie. That was the beginning of The Look.

I am not a has-been. I am a will-be.

In Hollywood, an equitable divorce settlement means each party getting 50% of publicity.

I think your whole life shows in your face and you should be proud of that.

You can't always be a leading lady.

What is the point of working all your life and then stopping?

I don't live in the past, although your past is so much a part of what you are that you can't ignore it. But I don't look at scrapbooks. I could show you some, but I'd have to climb ladders, and I can't climb.

Here is a test to find out whether your mission in life is complete. If you're alive, it isn't.

I wish Frank Sinatra would just shut up and sing.