MILAN, Sept 29 — The Gucci collection that opened Milan Fashion Week on September 21 was a hallmark show of the spring season. Yet one model, swathed in dusky pink silk and clutching a branded fan, had made her runway debut only 48 hours before.

Elibeidy Danis Martinez, a doe-eyed 19-year-old from the Dominican Republic, first set foot on a catwalk when she closed the Burberry show in London on September 19. Three days later, when she made her way onto the ramp at Prada, she had pulled off a triple casting coup, walking for three of the most powerful and high-profile brands in the fashion business.

It’s a heady beginning for a career, catapulting Martinez straight into the ranks of Vogue’s "new faces to know”. And it is increasingly common. The sped-up cycles of fashion don’t apply only to clothes, but to the women (or girls) who wear them. In the digital era, the pressure is constant for brands to feature new faces each season, the better to make themselves look new, too.

Which, in turn, means that a coterie of global casting directors — the individuals responsible for selecting the Next Big Thing from modelling agency books and putting them on the runway before anyone else — are becoming more important.

Barbara Nicoli and Leila Ananna, long-term collaborators with Gucci and Burberry, are two of the most influential agents. (Other names include Piergiorgio Del Moro, James Scully, Anita Bitton and Ashley Brokaw.) Each season for more than a decade, the two women have cast as many as 100 models per show for the two luxury powerhouses, up to a quarter of whom are likely to be faces no one has seen before on the catwalk.

Shuttling between Milan and Paris, they have also worked with Saint Laurent, Versace, Emilio Pucci and Diane von Furstenberg; this season, they will have cast shows for a host of smaller brands, including Peter Pilotto, Marco de Vincenzo, Acne and Arthur Arbesser. Among the models Nicoli and Ananna had a hand in introducing to the world are Karlie Kloss, Abbey Lee, Edie Campbell, Kiki Willems, Marjan Jonkman and Grace Hartzel.

Long gone are the days when brands looked for a standardised female aesthetic: Reed-thin white mannequins whose personalities were largely kept out of the spotlight. Today, with the demands of the digital age and new customers in new markets, the ground shifts constantly when it comes to both the identity and the values that companies need to project.

So while creative directors come and go, casting agents like Nicoli and Ananna remain entrenched on the fashion front line, filtering and shaping perceptions of beauty and its personification.

"Certainly there are no supermodels anymore, like Naomi Campbell, Claudia Schiffer, Cindy Crawford. That era has gone now,” Ananna said. "Sure there are beautiful stars, rising and falling all the time, but few, if any, girls will ever reach those heights again. Social media can make careers soar, but it can also overexpose them. After two or three seasons plenty have disappeared, and some of the biggest faces simply burn out. Ultimately, fashion will always be searching for something new.”

Elibeidy Danis Martinez, a new model, on the runway for Gucci during London Fashion Week, September 21, 2016. Martinez made her very first season of walks for three of the most powerful and high-profile brands in the fashion business. — Picture by Gio Staiano/NowFashion via The New York Times

That is where they come in. Casting often ends up being a year-round process for shows, according to Ananna, who is 42 and originally from Paris, as agencies continually send girls for consideration, usually scouted straight from the streets of their hometowns.

"The major advantage of casting for the biggest houses is that it gives you the best possible 360-degree view of the marketplace,” said Nicoli, 44, an Italian who began her career as a backstage dresser in Milan after earning a doctorate in political science. "Agencies from even the most remote countries have you on their mailing lists.”

The demands are such, she added, that: "most casting directors now work in teams rather than independently. It can be incredibly difficult to carry the stress and client needs on your own. Leila and I might do the same job, but our different personalities and tastes, and healthy exchange of opinion, unquestionably make us better at our jobs.”

The priority, they said, is always to find and to introduce faces unseen by anyone else. Models like Martinez go on to be cast elsewhere, of course (in her case, Prada is a client of Brokaw), but brands want the kudos of being first, and are willing to pay for results.

"Barbara and Leila have a very international point of view, and are always able to combine established models with a cool new crew of girls,” de Vincenzo said after his show in Milan.

Yet Burberry, Gucci and Jil Sander all declined to comment on the record about their model searches, sensitive to revealing the marketing puppetry that goes on behind the scenes.

Both Nicoli and Ananna, for their part, say it is hard to describe the magic formula involved in finding a face to launch a thousand advertising campaigns. The balancing act wobbles among a girl’s age, shape, skin colour and personality; a client’s creative vision, budget and timing; and the public mood. And, increasingly, a model’s presence on social media plays a role.

"Obviously the idea of casting a model and considering how many Instagram followers she had was unheard-of five years ago, but models who have built their own powerful brands? That is very important to lots of fashion houses now," Nicoli said. "I think it was Riccardo Tisci at Givenchy who started that trend. I wouldn’t say whether it was a good or bad thing, but the Gigis and Kendalls are a need of the market we live in at the moment. And for some, it clearly works.”

Personality comes into play, too. "They need to be a certain size, obviously, but Alessandro Michele at Gucci, for example, is far more focused on who a girl is and the fit of a look, rather than how tall she is or her hip size,” Nicoli said. She also acknowledged: "Of course, many will work purely on measurements. Last season, when we cast Céline, we made 200 cards with digits on, nothing more, and that’s what we took to fashion week.”

When it comes to diversity on the runway (or the lack of it), however, the two became a bit defensive — indicative, perhaps, of the sensitivity of the topic. "I feel as if it is more of a spotlight issue in the US than Europe, and I think in Europe we are further behind on tackling the diversity topic,” Nicoli said. "But I would cast a beautiful model, whatever her colour. Black, white, yellow, blue or red; a model is a model. But different brands have different specifications for what they want. That’s the reality we work in.”

Ananna said she agreed, noting that she thought the situation was improving as scouts look for more variation in models like Martinez, who is black, and who, for her fourth show, walked for Jil Sander.

"Who is she, she is gorgeous,” one editor whispered as Martinez made her way down the runway. "She has a such a commanding presence. She’s clearly going to be a star.” — The New York Times