Casting is about respect. There is something disrespectful about casting an able bodied person in a disabled part. A deaf actor coined it when she said "deaf is not a costume"
Hmm. Tricky one. Acting is a craft. The craft of being utterly convincing as someone you are not, with characteristics you don't have, behaving in a way that you wouldn't, for reasons you can't fully feel because their situation isn't yours and their feelings and experiences aren't yours. So you pretend and you pretend so well that you make people believe it. That's acting.
So take Daniel Day Lewis in My Left Foot. He won an Oscar because he played the very challenging role of a man with Cerebral Palsy so well.
Actors of that calibre with cerebral palsy will be hard to find, because actors with CP are hard to find in general, and actors of that calibre are hard to find in general. What is more important in the making of a brilliant film - the best disabled actor they can find, or the best actor they can find full stop?
Obviously an actor with CP doesn't need to act the CP, because that's already who they are. But can they act well enough full stop? Shouldn't the job go to the best person for the job, ie., the best actor?
After all, that's the job. Acting. The job is not having Cerebral Palsy. The acting is not the secondary requirement here.
Same argument for the deaf person. Deaf is not a 'costume' of course, but in the case of acting it is a role. And in all roles there is an element of having to take on the characteristics of another person. in this case, the role of a deaf person. You can find a deaf actor, I'm sure, but can you find a really good deaf actor? Would a non-deaf but completely brilliant actor make a better job of the role overall, than a deaf actor who was just an average as an actor generally, even though they were completely believable as a deaf? What if they weren't especially believable as the character in other ways? Because again, deafness is one aspect of the character, it's not everything about them.
If a deaf person took the role of a deaf person having a MH crisis, or being Autistic, would a person who actually does suffer with their MH or who is autistic have the right to say Mental Illness is not a costume? Autism is not a costume?
I'm not saying don't look for characters with specific disabilities, just don't make that the most important criteria in casting them.