Our first advert in PCR, for a student production to be shot on video, got us over five hundred actor's CVs and photographs. The first time you cast anything you feel so flattered that anyone actually wants to be in your film that you cast them regardless of their actual suitability for the part. Same applies if you ever get anyone half famous. This is the purpose of casting - to put the right face, voice and mannerisms to your words on paper. Unless you achieve that at this stage your film is doomed before you've even shot a single frame.
Flexibility is creativity.
Stubbornness is not creative. Kubrick did things the way he did them because he could. Change the characters to fit the most promising actors. You will still be able to tell the same story. Your characters are the agents of your story, they are not your story.