I agree that the idea is to maintain the director’s intent, and that a certain amount of growing or settling after a show opens isn’t necessarily in conflict with that goal. Actors are human, and the stage isn’t a film screen where every second will be exactly the same each night; that’s part of the joy of live theater. That said, whether or not a change is for the “better” is pretty subjective. Sometimes a small change made by one actor getting more “comfortable” can lead to a chain reaction of other actors making their own changes in response. In practice, you just need look carefully at each specific situation, and use your best judgement as to whether the director would support that change or not.
Often I try to take some artistic notes during rehearsals for just this reason, so I can look back at key directions. Ideally, the director and I chat briefly before they leave about what moments I should specifically keep an eye on, and where they think further development would be OK. I’ve also been in situations where I’m communicating with the director after the show opens, and I can specially ask them if they think it’s a problem for, say, James to smile more in his Scene 2 monologue.
Giving actor notes can be tough. I do think that a quick chat, in private, is the way to go. Tailor your approach to the specific actor, and don’t take a negative response personally. I like to watch for a few performances before I give a note (unless it’s something like a safety issue) to make sure it’s not just a fluke. I also try to
ask the actor about the moment, and see if they volunteer a reasonable explanation for whatever they’re doing. Let them know what your concern is, and if they’re ornery about the note, ask them to at least think about it or try it and then talk to you again. Ask if there’s anything you can do to help…sometimes a preset or Qing issue can be at the root of a change. If it’s blocking, you can always blame lights (there’s a Special for that scene; if you move it to DSR you’ll be in the dark!).
Here's another thread related to this topic:
http://smnetwork.org/forum/index.php/topic,912.0.html