Designers usually hire a casting director whose sole job is to cast/help to cast the models for the show. The level of involvement of the designer in this process depends - some are a lot more hands on while others less so, but in both cases there is an understanding that the models generally should reflect the aesthetic of the label and the collection for that particular season. There will be a ‘look’ the designer is after that season and maybe a girl that epitomizes that look (an example is the opener/closer of Marc Jacobs this season - Ruby Jean Wilson). You might notice that some brands have a ‘type’ of model that they always use but for designers that are less.. shall we say.. “directional”.. they will often be heavily influenced by the look that is in that season. Everyone is sure to remember the doll-faced trend that was popular circa 2004-5 when Gemma Ward, Lily Cole et al dominated the runways and led the way to designers booking similar looking girls. So, definitely, what is “hot” right now is a big influence on the girls that get booked by designers at any time.
You’ve probably heard of the week before fashion week when all the models in town go on show castings. This is the models opportunity to meet with casting directors, designers, stylists. Established models tend to not cast as designers know them and will direct book them from their agency however for new faces, and even girls in their 2nd, 3rd, 4th etc season, this is the most important time. A lot of the big brands opt to do request casting which means they see less models and only models who are invited - for a new face to break into this they need to have hype surrounding them or someone big/important supporting/backing them.
So, although there are ‘so many (models) out there’ in reality I would say a large percentage of the spots for a model in a ‘good’ show are already taken by a) designers favourites (e.g. Hanne Gaby has walked something like 9 consecutive seasons for Alexander Wang) or b) casting directors favourites. Top casting directors like Russell Marsh, James Scully, Ashley Brokaw etc all have favourites and a certain ‘type’ of model they prefer. For a new face, impressing these big casting directors is really important because they will often work with more than one designer and across the 4 cities. James Scully casts ODLR, Carolina Herrera, Jason Wu, Derek Lam in NYC; Ashley Brokaw casts Prada, Miu Miu, Balenciaga… A good start (e.g. impressing one of these people) in New York for a new face model often leads to great things in Europe because they cast the big shows in Milan and Paris too.
Although many designers use casting directors, a lot of fashion houses, particularly in Europe, actually do inhouse casting.. Lanvin, Hermes, Jean Paul Gaultier.. Karl Lagerfeld casts all his shows.. this can either be really good or really bad. Personally I feel unless the designer has a really strong aesthetic it can become a bit sloppy and lacking cohesion. I think the best kind of casting is when the models in the show fit the collection, a great example of this in my opinion is Prada’s casting - the show is always cast based on the collection that season and there are numerous cases where girls who have landed campaigns the season prior are not booked because they don’t fit the look.
This was a really long answer - hope it makes sense??
As for Calvin Klein casting - I didn’t like it because there was no diversity whatsoever (no models of colour). I wouldn’t say I was disappointed though because I never really like CK’s casting anyway, it’s always a bit bland..