I'm in the Wilder Pageant, an annual show in Walnut Grove, MN, and we go through that every year. On any given rehearsal night we're going to be missing at least 5 cast members, and it's usually with no notice. Our advantage is that we do basically the same show every year (it's a historical pageant) so we have cast members who can step in at a moment's notice, but still... I don't envy the director.

There are always conflicts. Things come up. You deal with them the best you can. Community Theatre is different in that this is a hobby for people. They have life and work commitments that take presedence. If they get ill, or a child gets ill, or a baby-sitter doesn't show up, or a work issue occurs, you need to deal with as best you can. However, these should be rare. The actors should know the expectations of them before they accept a role and then agree to abide by those expectations. Now if it is not rare, say a work or school schedule that was not revealed in the audition sheet, then there is a problem and re-casting may happen. You (the director) should not feel bad about this since the actor was not upfront and tried to conceal the true circumstances. Not good! And that behavior should be considered when they audition in the future.