I mean, if you are directing actors to do one thing and then directing them to do something else entirely because the one thing you wanted them to do may not work, then you are just shattering their confidence in the project.
Donner likes what he does and enjoys being on a set. He doesn't have to do it, its not like he has to make the rent. When I get to be his age and still enjoy what I'm doing like him, I'd be lucky.