Acting isn't just about memorizing lines, and then reciting them back to an audience during a performance. No, the lines merely dictate the story. They tell the tale of what the playwright has written in his/her original script. It's the little stuff, like mannerisms, inflections, expressions, and the actor's physical interpretation that make a story come to life.
And that's what's so great about rehearsal. It's the time when actors come together, and play with these things. We experiment. We create. We see what works and what doesn't. We take the lines, commit them to memory, and then have fun playing with the details.
It's like a sculptor working with clay. He shapes part of his creation, looks at it from all angles, and then decides he wants to make things bigger. So he adds more clay, molds the shape further, and then steps back once more. Maybe now it's too big. So he removes some clay, and takes another look. When he finally has his creation the way he'd like it to be, he puts down his tools, picks up his sculpture, and puts it carefully into the kiln. Hours later, he takes the hardened object out of the hot furnace, lets it cool, and can finally prepare it for presentation.
Metaphorically, that's exactly how the rehearsal process plays out in theatre. With the help of a good director, each actor adds and subtracts from his or her performance until everything works together in dramatic harmony. Then, when the cues are set, the curtain rises, and the audience grows silent, a theatre production is ready for presentation.
Theatre is one of the oldest professions in the world. And it remains so because creative individuals long to play, create, and imagine. Which, in turn, brings eager audiences together with a deeply-rooted longing of their own... to be engaged by that collective imagination in front of them.
Without actors sweating the small stuff in rehearsal, a play is nothing more than a script. And without visual interpretation, a script is nothing more than a good book.