How do you learn your blocking? The same way you learn your lines: by practicing it over and over.
I am surprised by how many people rely strictly on rehearsal time to learn their blocking. If your blocking is fairly simple and you make clear notes in your script, this is a perfectly do-able approach. However, if the blocking is at all complicated or there is simply a good deal of it, it is difficult to get it down quickly if you go about it this way. And you do want to learn your blocking quickly, because until you have it down, you can’t do much else.
How quickly do you need to learn it? Let’s say we block the first scene on Monday night. The next time we run that scene, there are apt to be some moments when an actor is confused about where he is supposed to be, or someone has come up with a better alternative than what was done on Monday. So the second night we work on the scene, we sort out all of these moments, and we should end that night with a clean run. And that should be the end to 95% of blocking confusion, and by blocking confusion, I mean the actor who says, “Oops, I was supposed to have crossed to the door four lines ago!” as he dashes to the correct place, or the actor who is surprised to find that he’s supposed to be somewhere other than he is.
This DOESN’T mean that once blocking is completed, it can’t change. It can. As you get into rehearsals, you may realize that you can change the blocking to make something funnier or otherwise more effective. Once you get familiar with the lines, you may realize that the timing is incorrect and the blocking needs to be changed to accommodate this new reality. As you learn more about your character, you have little epiphanies about what your character will do that you couldn’t have back in the first week of rehearsal when you blocked the show. To not change the blocking for any of these reasons would be silly, and you should definitely do so.
So yes, that means you have to unlearn what you learned to begin with and replace it with something else. The nature of the beast, I’m afraid. The point is that you need to quickly memorize your blocking be doing whatever you’re supposed to be doing at whatever point in the rehearsal process you are. If you don’t, then your rehearsals stop being about making discoveries about your character and are just about figuring out where you’re supposed to move when. Just as rehearsals when you haven’t fully memorized your lines but you insist on running the scene without your script are just about you trying to remember your lines. It’s a waste of everyone’s time and disrespectful of those you are working with.