Penelope Cruz flaunted a remarkable working-mum quality when we met her in Loewe’s Madrid headquarters in June, slightly more than a month before she gave birth to daughter Luna Encinas Cruz, her second child with fellow Spanish actor and husband Javier Bardem.

On a sizzling high-summer afternoon, the heavily pregnant actress turned up with her sister Monica. Even in a casual ensemble of T-shirt and maternity jeans, with little make-up other than her signature smoky dark eyes and nude plump lips, the expecting mum radiated her usual superstar aura.

She was buzzing with excitement over her new collection for Loewe – Spain’s oldest and most prestigious luxury brand – which she worked on with Monica. The project took months to prepare and finally saw the light of the day for the first time. The capsule collection designed by the sisters will hit stores next month.

The collection, featuring cross-body bags with exterior pockets and other small leather goods in a rainbow of colour combinations, reinterprets Loewe’s classic silhouettes for a modern-day lifestyle envisioned by the Cruz sisters. Think a remodelled Amazona, featuring multiple pockets, attached with shoulder straps and painted brown with orange polka dots.

“We’ve worked with other brands around the world before, but the Loewe collaboration is really close to our heart,” says the veteran actress, referring to her previous collaborations with the likes of Samantha Thavasa, Mango and Agent Provocateur. “It’s special because we have known the brand for so many years. We grew up buying Loewe bags, and Loewe is really ‘the’ Spanish brand.”

As Spain’s most famous actress and the Academy Award-winning star of Volver and Vicky Cristina Barcelona, Cruz is a perfect collaborator for Loewe. Even before the launch of the capsule line, which she actually has a say in designing, the actress had already starred in various campaigns for the luxury brand. The capsule collection is an organic extension to Cruz and Loewe’s long-term partnership.

“Even before we started the collaboration, Penelope, and especially her films with Pedro Almodovar, have been a huge [inspiration] for me,” says Loewe’s former creative director Stuart Vevers, the mastermind behind the Cruz collaboration before moving on to the helm of American accessories brand Coach in June.

“That kind of flamboyance, bold sensuality and the fact she’s probably the most famous Spanish star of all time ... it was natural to reference her style and beauty.”

While celebrity endorsements for luxury fashion brands are practically the industry norm, it’s now taken a step further, with celebrities such as Cruz are invited not only to pose with the products, but to actually design them.

Past high-profile collaborations between style icons and luxury brands, such as Kate Moss for Longchamp and Alexa Chung for Mulberry, brought not only publicity hype but also a sales hike.

The Chung for Mulberry collection launched in 2010, for example, was credited for the British luxury brand’s almost 60 per cent increase in sales across the label’s 40 British stores, compared with the same period in 2009.

Vevers was obviously hoping the same glory would follow after the Loewe-Cruz collaboration.

“It helps [the brand] reach new audiences because you are telling different stories,” he says.

The experience also helped Vevers’ own creativity and interpretation of the brand.

“Normally, as a designer, I’m more often imposing my ideas on the products. But this time, I had direct communication with a remarkable woman about what she wants in her dream bag,” he says.

“It’s a combination of my instinct for the brand and what Penelope is looking for in the function of a bag.”

To keep the collaboration genuine, Vevers says the Cruz sisters were involved not only in brainstorming, but also decision-making.

“It a l l started with a conversation with Penelope and Monica,” he says. “They wanted to play with proportions for the Amazona, add pockets and shoulder straps – a generally more casual [approach] to the bag, and I think that’s how people dress today.”

Working as a team for almost all their design projects, from Mango to Loewe, has been a blissful journey for the Cruz sisters.

“I’m happy we’re working on the project together; it’s easier between sisters,” Monica says. “We also get to do what we are passionate about. It has actually gone from a game we play when we are little to something we do professionally.”

Going through the trophies of their creativity, Penelope names the orange polka-dot style as one of her favourites from the Loewe collection. “When I suggested polka dots, I wasn’t too sure about it, but Stuart loves [the idea] as the print pays homage to Spanish culture and heritage,” she says. The idea of a more functional twist to the iconic Amazona bag comes from her working-mum experience. “I want big bags that have a lot of pockets,” she adds.

The latest adopters of celebrity guest designers are exploring a natural transition and relevance between collaborators and are involving genuine creative forces on a less superficial level.

For example, supermodel Anja Rubik had been a campaign face for seasons before designing a capsule collection for Italian shoe brand Giuseppe Zanotti, available at its online store. Actress Sarah Jessica Parker channeled her beloved on-screen persona Carrie Bradshaw from the Sex and the City series to design a collection for Manolo Blahnik, due out in February next year.

Commenting on the Anja Rubik collaboration, Zanotti describes the process as a “pleasurable and exciting game”. “[It was] a game between myself as a designer, with my experience and sensitivity for women’s shoes, and Anja as an iconic woman, top model, the muse of some most important fashion houses,” he says.

“It’s also an analytical study of the ideal shoe for a woman on one side and on the other side the ideal woman for my shoes. Anja’s style, taste and sense of proportions after nearly two decades working in the fashion world are recognisable and distinct.”

While a slew of luxury brands, marketers and publicists are jumping on the bandwagon for celebrity guest designers hoping to reach new customer segments, there are those who remain sceptical about such collaborations.

Ultraluxury houses including Hermès and Cartier have rarely used celebrities in their official campaigns. Instead, they rely on their heritage motifs, highlighting their products as the star.

British shoe designer Rupert Sanderson, who counts the likes of Kate Middleton and Gwyneth Paltrow as fans, is collaborating with fellow fashion designer Antonio Beradi for a collection of evening shoes for spring/summer 2014. Sanderson also sees the celebrity craze differently.

He says it is fantastic to see celebrities wearing designer shoes, but having celebrities design products could be “a bit dangerous”.

“I believe we should all stick to our own crafts. [I wouldn’t fare well] if I were asked to act in a film.”

Sticking to her own craft, Penelope Cruz says acting is her priority, despite the multiple offers from international brands. “Acting is my main job. In fashion, I’m more like a guest. We don’t walk around like we are professional designers, but we are here to learn.”

Vevers, who collaborated with Japanese avantgarde streetwear brand Junya Watanabe and just launched their autumn/winter collection, sees a difference between working with a celebrity guest designer and a designer by training.

“With Junya, we wanted his take on Loewe and, even in a broader sense, his take on luxury,” Vevers says.

“Junya, who has his own collection, was more involved in the production side compared to Penelope. He knows how it works. He came to Madrid twice for the fitting. With Penelope and Monica, it’s more about sharing ideas.”