What is right and what is wrong

I am writing this, i guess, simply just to see who else has been in a situation similar to mine, who can relate and perhaps, what certain directors may have to say on the subject.

I was cast in a play not long ago. A great play. A play of which i have always been fond of, and, passionate about.

I get cast in a supporting role, and just feel privlliged enough that i have the chance of being in this play. Until i meet the person who is to play one of the leading roles.

I was shoked at his dreadful reading ability, his inability to express the true meaning of the character and his obvious lack of preparation, all of which, i am hoping will improve.

Has anyone ever been in that situation. I'm sure you have.

Directors - have you ever cast someone for a particular role, a leading role, and then wished you hadn't. Realised that this perhaps wasn't the right person for the part, and that perhaps someone else in the cast had the "right goods" for the part?
What did you do about it? Have you ever changed things around? Or are there any of you out there who would admit to not changing for whatever reason, and perhaps even regretting it?. If so - why?. Why did you do it?

Who can relate to this?. Who has ever felt that the casting in the play they were in was unjustifiable?.

Now, i know this is amateur theatre, but we all strive to be professional in our work.
I have worked with some outstanding talent, and talent fitting for the role. But sometimes, just sometimes......it all looks a bit miffed.