Presumably, the ones that opened their mouths and spoke with the voice of Satan were put on the shortlist.

They would select for skills needed for that film eg running underfoot on cue. They would also likely be trying to match cats with doppelgangers.

For example, here's Ian McKellan talking about 4 identical cats on the set of Apt Pupil each of whom had particular skills needed for the scene where he kills a cat:

This was the day of the Cats - there were four identical ginger toms. The docile cat which could be thrown and shaken and swung in the air and just miaowed for more. The cat which could move to order. A third who could eat on cue. Then there was the feral cat only to be approached by its trainer, wearing an armoured glove! All feline manouevres were achieved by the bribe of edible paste, overseen by an animal rights official, who was on set to protect the 4 pussies.