During previews the creative team will of often tinker, add, cut, tighten and change things in a show, sometimes major, sometimes minor, sometimes on a daily basis. Once they are finally satisfied (or at least not too unsatified) they "freeze" the show and they won't make anymore changes to the production. This gives the actors a chance to settle into the final version of the show (and not have to worry about new lines, new blocking, new choreography, new costumes etc ....) before the critics show up. Depending what shape a show is in when it begins previews, the creative staff may stop tinkering and freeze it after just a week or two, or if a show's having problems, it may not be frozen until the day of the first critics' preview (usually about a week before opening).
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
[http://margochanning.broadwayworld.com/]
"The Devil Be Hittin' Me" -- Whitney
