Star casting is discussed as an artistic decision and operates substantially as a financial one. A name attached to a project changes how much money can be raised before anything is shot.
Films are frequently financed before they exist
Independent and mid-scale productions are often funded by selling distribution rights territory by territory in advance of production.
Buyers commit against a package: a script, a director, a budget and a cast. They are forecasting from that package alone.
A recognisable performer makes the forecast tractable, because the buyer has seen how similar films performed in their market.
Those commitments are then used as collateral to borrow the production budget, so the cast is doing work in a bank's assessment long before a camera is switched on.
The value is specific to a territory
Recognition is not uniform across markets. A performer who guarantees a strong sale in one region may be unknown in another.
Sales agents therefore assemble casts to cover territories, which is why casts sometimes look assembled from unrelated traditions.
This also explains roles that appear too small for the performer playing them, since the presence in the cast list is what was being purchased.
Backend deals move the risk
A performer can take a reduced fee against a share of revenue, which lowers the cash a production must raise upfront.
These arrangements are heavily negotiated, since the definitions of the revenue being shared determine whether the share is worth anything.
The alternative, a large guaranteed fee, transfers all the risk to the production, and only films confident of recouping accept it.
The reliability of the mechanism decayed
For a period, certain performers were regarded as guaranteeing a floor of attendance regardless of the film.
That reliability weakened as audiences began selecting by property rather than by performer, and recognisable material took over the role that names had played.
Performers now attach to franchises partly to rebuild the standing that opening films on their own once conferred.
Where the old logic still holds
In presale-financed production the mechanism is intact, because buyers still need something to forecast against and a name remains the clearest signal available.
It also holds for prestige projects, where a performer's involvement signals to festivals, distributors and awards bodies that the film is serious.
The billing itself, whose name appears first and whether it sits above the title, is contracted in detail precisely because it carries this value.