It’s not every day you get locked in a lingerie store in London’s banking district with an A-list actress. But then Penelope Cruz isn’t your average star; she lives in Madrid, not Hollywood, for starters, and is the antithesis of the current wave of celebrity-turned-designers (who pop their name on a bottle of perfume or label in return for a handsome cheque with little or no shame).

Cruz, along with her sister Monica, has been quietly beavering away on L’Agent, the more competitively-priced line of Agent Provocateur since its launch over two years ago. L’Agent has been a huge success since day one, boasting four standalone stores (one in London, Los Angeles, New York and Montreal), as well as hundreds of stockists around the world. What’s even more interesting is that L’Agent never existed without the driven and passionate Cruz sisters, making this a more genuine collaboration than most.

“All of this is like a dream come true,” says Cruz inside the L’Agent store in Spitalfields. Resplendent in a black, off-the-shoulder dress, the ridiculously sparkly diamond-encrusted hoop earrings she has chosen to wear serve as a constant distraction.

Monica, Penelope’s younger sister by three years, has joined us a smidge late owing to the shopping distractions of the adjoining market. But it’s these sorts of markets that feed the sisters’ inspiration for the collections that they create with Agent Provocateur creative director Sarah Shotton – along with ballets, films, books, images and paintings. “Sometimes we bring things to show the other one, many times we [Monica and I] come home with even similar things, or very similar ideas,” reveals Cruz of the design meetings.

Do they ever disagree? “She [Monica] and I have a really similar taste, we go always in the same direction, if we don’t agree we find an in-between that works” says Cruz.

The 41-year-old actress, who has two children with fellow actor husband Javier Bardem, has design aspirations that pre-date her film career. “My sister and I, we used to do this as a game, of getting locked in the bathroom with books and magazines and designing on top of outfits, and ‘which one would you love to have?’ and we would fight about which one, and really like play designers, for so long.”

And they really are hands-on. “We see every single step of the way – everything. Like if there is a bow here or not; if the black goes here or the black goes there; we touch everything and see everything and then we see it on a woman’s body. Everything, this is not like a pretend thing, this is a real collaboration.”

A modest Cruz is the first to admit that her foray into fashion is “from a very humble place”. She says: “We are learning, but they are giving us an incredible opportunity and now we’ve worked with like, five different companies around the world but this is our most important project because it was born with us, and we are there every single step of the way.”

Indeed, L’Agent was born out of a conversation between Monica – a successful actress and dancer in their native Spain - and executives at Agent Provocateur after she was picked to pose for the brand in 2013. Already firm AP fans, the official proposal came through and the rest is history.

The union has also marked another first for Penelope in that the short and very cheeky viral videos to accompany two collection launches facilitated her directorial debut. In the first one, an eight-months-pregnant Monica starred. “She looked amazing, and that was important too you know, to put a woman that is pregnant there and that looks like that. And you can find things here - even if you are eight months pregnant, so that’s really important, that it’s not just limited.”

The Cruz siblings might be blessed with beautiful bodies, but they’re more than aware that women come in all shapes and ages. “It’s important for us to have a variety of sizes, you know this is not limited for bodies of a either very young or very fitted, and I try to put that down in the campaigns. Of course, there were so many beautiful women in the campaigns but not all of them had the same size and that was really important for us.”

Price is also something they feel equally democratic about. Apologising to AP brand reps, Cruz lets slip: “I think that the quality and the coolness of the design is as high as Agent Provocateur – sorry I always have to say this because it’s true – but the quality is amazing but the price is really affordable so we can talk to a younger public also, and this makes me really happy.” Bras start at a reasonable £40 with many going up to an E cup; briefs start around £25.

Cruz is delighted to share that swimwear will be debuting in November, complete with vibrant chiffon cover-ups alongside bikinis and one-pieces. The sisters’ current favourite piece is a black lacy body called Brigit (£95, below), which they purr over as it takes centre stage inside the petite boutique.

Underwear aside, I ask Penelope what her favourite piece of clothing of all time is. Expecting her to recall a gown that she’s worn to the Oscars or similar, it’s in fact a very private piece of clothing. “[John] Galliano made my wedding dress so this is very hard to beat” she reveals. “He and Alexis [Roche, a stylist and Galliano’s partner] were amazing, what they did for me, and he did something that has not been seen, but is really beautiful.”

Clearly thrilled to be able to do a little designing of her own she says: “Sometimes I look at the store and I feel like I am still in the bathroom with her, hiding, playing with dolls and dressing them but this is real, you know, so we are very happy about it.”