This happens particularly in scene class, but I’ve seen it happen in regular rehearsals, too. In class, I assign a scene of two to three pages. The actor gets the script and reads it. Now he knows the gist of what happens in the play and has a feel for who are the bad guys and who are the good guys. He has a visceral response to what sort of person his own character is. Fine. That’s enough, right? Now he’ll just work on the scene.
Sorry, but it isn’t nearly enough.
Working on a single scene requires a lot of the same investigation into the character and his background that working on the entire play demands. You can’t understand your character in isolation. You’ve got to know what happened in the scene before the one you’re playing before you can begin to understand how he feels in this scene. Background information that is revealed in scenes before and after yours may help to explain something that happens in your scene. A comment made in Act II sheds light on something he said in Act I.
As for rehearsals for a full production, it’s not enough to encounter the play when you are working on it with the rest of the cast. It’s not enough to read it for the purpose of memorizing your lines. Plays are littered with clues that help you to understand your character, and at some point I’ll talk a bit about how to find them and put them together. The point is, you have to look for the clues, and you can’t do that particularly well when you are running a scene or memorizing your lines. Yes, you’ll discover some things when you do, but it won’t be enough.
When I act, I am actively mining for information about my character throughout the rehearsal process (and throughout performances, for that matter). By “actively mining”, I mean that I am paying close attention to everything that is said, and everything that I read, to see if I can understand it on a deeper level. There isn’t a magic number for how often you should read a play, but I probably read the ones in which I have a large part at least 80-100 times. You don’t have to read it that many times, but I hope it suggests that more than a half dozen times is required to really get the most out of it!