In theatre, early rehearsals (the first two weeks or so, depending on the theatre) are done with the cast reading their lines from the script.

Then comes the dreaded day when rehearsals go off book. That means, by then everyone is supposed to have their lines memorized. Typically, actors who have few lines do indeed have them memorized, those with major roles tend to go through a major struggle on the first off book night. See here how they deal with it.