Naturalism began as a late nineteenth century revolt against artificiality, and as part of an actor's training, it remains a useful tool for exercising the sincerity muscle. It is important to remember that, as my former colleague, Alain Schons, once rightly insisted, Naturalism is a style that evolved within a particular historical and cultural context, and as such it is just as "artificial" as any other theatrical style.
In other words, naturalism carries its own supertext. And it is the biggest lie of all, because there IS an audience out there, and the actor is asked to expend so much energy pretending there isn't. And the audience itself, if it isn't feeling embarrassed by being turned into a voyeur, is surely insulted by the overt denial of the obvious!
How could it be anything but destructive to base an art form on the wilful concealment of a lie? Illusions, like the wooden sword, are most magical when all present, actors and audience, are active participants.
There is a built in paradox to be found in many approaches to naturalistic acting: they often involve a great deal of analysis. You know, the usual suspects; objectives, circumstances, character biographies, that sort of thing. All very useful as a shared vocabulary between director and actor, but they tend to push the actor up into his head. They tend to encourage the I-myself (system two) at the expense of the Me-myself (system one). The result is a very constructed "left brain" style of acting which has its application when playing strategically motivated characters, but lacks the spontaneity that one would normally associate with a simulation of everyday life.
Actors, when confronting the paradox of naturalism, often retreat into a boring sub-minimalism. But there is no escaping their supertext! You so often see the wheels turning behind the eyes of such actors, under the glaze of their self consciousness. They seem to be screaming out "I am so naturalistic; I am so naturalistic; I am so naturalistic." And they are. But they are not natural!
I have my own pragmatic, perhaps rather cynical, definition of naturalistic acting, which students seem to grasp immediately. I present it as a kind of thought experiment: If a scene should accidentally be overheard by a casual passer-by, perhaps standing on the other side of the studio door, and she is not able to recognize it as theatre, but rather believes herself to be witnessing a real event, then that is naturalistic acting.