There is a great temptation to do things half-baked early on in a rehearsal process. Often in the first two weeks or so, (sometimes more), there are lots of short cuts, and little energy. I feel justified in pointing this out, because if I am not careful, I can adopt the same attitude.
The reasons for this tendency early on are simple; it does not feel in anyway like a production. Consider the first week or so of rehearsal in the average show.
There is often no set, only an empty stage. No one is in costume. There is the cumbersome nature of walking around with the script in one's hands. You have the constant changing of the director's mind in regards to blocking, which is almost inevitable early on in a production. All of these factors conspire to make one feel an audience is at least 100 years in the future. Ergo, lazy line readings, slow, meandering crosses, and an overall emotionless presence on the stage are commonplace. In other words, few people are performing.
Of course, even the best of the best need that time to acclimate themselves to new stages, new scripts, and new cast mates. They are not at the top of their game from the first instant. It is, however, the best of the best that will use every moment that is available to them before opening night to get into that all-important groove. From day one they are putting feeling into line readings, crossing the stage with energy and purpose, and seeking to interact in a convincing manner with their fellow actors. Though it is rough, it is a performance.
What you do with your character and line readings on the first night will differ quite a bit from what you present on opening night. (One would hope, anyway.) The idea, however, is to be presenting something all the time. Never simply go through the motions. Your mind, body, and spirit are not exempt from inertia. If for weeks you have been phoning it in, waiting for the set to be complete and your costume to be ready before you start, you will find it twice as hard to kick it all into gear when you need to. Contrary wise, if you start off in motion, (as Newton said), your performance will find it much easier to stay in motion. With hard work and luck, by the time that motion carries you into opening night, you can take off and fly with it.