The proper relationship of the text and the performance is often controversial, and different styles of theatre handle it differently--as do, unfortunately, different actors. But let's take a step back for a moment, and look at it in terms of the questioning of text--written words--by those who like to use spoken words as a stand-in for electronic media.
For instance, classicist James J O'Donnell (quoted in a book I'm reading for review elsewhere, called Distracted, by Maggie Jackson): "Is it not strange that we take the spoken word, the most insubstantial of human creations, and try through textuality to freeze it forever, and again, try to give the frozen words of those who are dead and gone, or at least far absent, control over our own experience of the lived here and now?"
Well, it might be strange if that were even close to true. For starters, while the spoken word has been "insubstantial" in a technical sense, at least until sound recording, the oral tradition of storytelling is hardly insubstantial in the larger sense. Stories that Homer recalled were told for hundreds of years before he wrote them down. Stories in the Native American tradition are still told, in living evidence of how the oral tradition works. (See The Truth About Stories by Thomas King, for evidence.)
But the real smasher of these sentences is performance, even theoretically. Apply what O'Donnell says to music, and you can see the logical problem. Music that is performed once is indeed "insubstantial"--that is, lost forever--unless the notes are written down (or the performance is recorded.) But once it is written down, according to O'Donnell, we are slaves to it forever, as it controls our experience of the here and now. Can't live with it, can't kill it.
It is certainly true that a piece of music, especially when heard repeatedly, and a book, and a play, can in some sense form our experience of the here and now, even beyond the here and now of the actual performance. That's why people associate certain songs with particular experiences, or play particular music to induce certain moods. Our view of the world is shaped in part by what we read, hear and watch--which sometimes means, by art.
The obvious example of how the fixed (the text) combines with the now is performance. Even apart from intepretation that consciously seeks to make a text more relevant to the now, or consciously shapes it for some other purpose, performance, by being in the moment, inevitably brings the text into a new time and place.
It's the best of both worlds. We can have performers, and all the people involved in a production, plus the audience of each particular performance, combine to create something that is alive in the here and now. While we also have the text of Shakespeare's plays, safely preserved, so that others may try to find and present its truth. Note that word: present.
For the here and now is not the last word in wisdom. Nor is the latest medium necessarily the best--compare for instance the books that physically last decades and hundreds of years, and that can be read by anyone who knows the language without assistance more complicated than eyeglasses, versus decaying diskettes and the multiplicity of formats and computer languages that has turned information storage into a gigantic toxic Tower of Babble.
Each generation finds newness in the text of at least some of the dead and gone. Voices from the past may be as important to our present as they are likely to be to the future. Only the arrogant, the deluded and the ignorant of the lessons preserved in some of those texts, would think otherwise.
Just as a given text can transcend its historical time, it may well be useful beyond a particular performance of it. There is an inherent tension between text (often composed in isolation by individuals) and performance (often resulting from the dynamics of many people working as a group), and it is healthy tension, like the tension that holds the structure of a geodesic dome together.
Besides, as a practical matter, while some actors and directors feel constrained by a text, many more actors (in my experience) are hungry for playwrights to give them words to say.