We all had dinner together afterwards, too. I’d changed out of the green dress and didn’t have shoes on because I couldn’t deal with heels anymore. There was a little balcony bit where we were, so I went out onto it. It was deadly quiet outside, but the stars were up and you could just hear frogs, and I could see everybody sitting together and it was the first moment that I actually had just for myself, to absorb it all. Then I had another little cry.

Tell me more about that green Oscar de la Renta gown. People have been speculating that you were paying tribute to your mum’s 1998 Givenchy by Alexander McQueen Oscars dress, the year she was nominated for Titanic?

It’s so funny, because I didn’t know that she’d worn this dress. It wasn’t intentional at all. As soon as the film wrapped, I thought, if this goes on a red carpet, I have to wear green, because that color is important to Liesl—she has these green stockings and green eyeshadow, which are her signature. This Oscar de la Renta dress was gorgeous and it made me so happy, but yeah, I had no idea that she’d worn something similar. Someone mentioned it to me the morning after and I thought it was hilarious.

Threapleton, 24, at the Cannes premiere of The Phoenician Scheme.

Pascal Le Segretain/Getty Images

A 22-year-old Kate Winslet at the 1998 Oscars.

Jim Smeal/Getty Images

And you’re wearing McQueen today as well.

I’ve always loved this stuff for as long as I can remember. I must have been about 11 when I went to see “Savage Beauty,” and I became completely obsessed with McQueen.

I think your mum was also very young, just 20, when she first came to Cannes. Did she give you any advice for your first time here?

She said, “Try not to be sick out of your nose. Just breathe!” I’ve been so busy while I’ve been here, I haven’t really spoken to her at all.

Winslet, aged 20, at the 1996 Cannes Film Festival.

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Threapleton at the photo call for The Phoenician Scheme.

Photo: Getty Images

Has she, or the rest of your family, seen the film yet?

No one who is close to me has seen it yet, but I can’t wait for them to see it. That’s another reason why the premiere felt so big—I was suddenly like, oh, wow, everyone’s going to watch this. The cat’s out of the bag now. And it comes out in cinemas so soon. I keep seeing the posters everywhere, and I just want to take a dive. I don’t really know what to do.