If time allows, I prefer to come into the first rehearsal with a broad familiarity with the text (meaning that I've read the play several times), but prefer to leave the actual memorization till we've blocked a bit. The reason for that is that I usually find that it's easier to remember lines as part of a whole, meaning in conjunction with blocking, business, and my scene partner's line readings, rather than as a single disjointed piece. Of course, that only works if there's enough time in the process to allow for it.
I've done many a play where I didn't even get the script until the first rehearsal. |