How do you handle the cast member who is quite certain he knows
who SHOULD have been cast in x, y, or z role, and who has no
qualms about coming up to the director after rehearsal to ask why
Joe Schmoe wasn't cast in the role, because after all Joe Schmoe
would be SO much better than the person who is actually cast in the role (while Joe Schmoe is standing within earshot the whole time).

Last night someone asked me that - and this is someone who has
asked me about my casting decisions before.

I think your reply has to take into account the real reason the person asked the question. When I first got into directing, I asked several directors why they made a certain casting decision. For the most part they told me what they could. I learned there are many reasons one person is cast over another. Often the choice is made on nothing more than the directors artistic feeling of who will work better in a specific situation.

I get a few questions from actors wanting a de-briefing as to why they were not cast or not cast in a specific part. I call every actor who auditions but was not cast and try to tell them why I did not cast them. Sometimes it is easy - you are too old/young - sometimes it is not easy - particularly when I have passed over this person several times or cast an actor with less experience. Sometimes it is just a gut feel who will work better. I have the advantage that we are a training theater and we seek out new and underused actors to cast.

For the inquiring mind who has to second guess the directors casting decisions, I have a pat answer: I will not discuss the strengths and weaknesses of any specific actor - it is not fair to them any more than it would be fair for me to discuss you with someone else. My casting decisions are not made in a vacuum - every actor has to fit their part and work in the ensemble. In making my casting decisions I considered A and B and C and D. I am confident that I have the best combination of actors I could assemble.

As an aside, one of the reasons I get frosted when an actor turns down a role after being chosen is that it changes the dynamics of the entire cast to substitute someone else. I have a box on the audition form that says "I will take any role or I want to be considered only for the role of ____________". In spite of this, I continue to be amazed at those who forgot they have a two week family vacation right in the middle of the run after I have offered them a part.

Personally, I don't think you should have to justify a casting decision to anyone. I've been directing for ten years and wish I could cast everyone who auditions but let's face it, acting contains a very high percentage of rejection. You won't always get the part you want. You were asked or hired to direct because the producing company had enough faith in you to get the job done. They obviously trust you to make the right decisions. Never forget that you are the boss and it is unethical and just plain rude for an actor to question your casting decisions. If they have an inflated ego, that's their problem and you shouldn't have to deal with that. All of this may sound dictatorial but you can never let anyone undermine your authority. That just opens a whole world of problems that you won't have time to deal with.