I'd say I am about 85% off book at this point. The process has been a little different for me this time, mostly by my own choice.
Normally a director assigns a deadline for being off book from the start. That didn't happen this time. As a result, though I still go over my lines, getting familiar with them has been a somewhat more organic experience. Just the nature of time and going to rehearsal taking the lead over studying. That ratio might not work every time, but it seems to be working this time.
The biggest problem is that Mamet writes in so many implied interruptions, and I think it's a weakness in the script. Instead of having someone speak, and write a new tagline for an interruption, and then a new tagline for the return, Mamet give's long speeches a whole lot of "I was thinking, NOW WAIT A MINUTE, I was just thinking about, HOLD OFF PLEASE, thinking about something, JUST WAIT."
And so on. It's distracting, it's more difficult for the other actor to memorize, because it's sort of an anti-line, and to me it's a bit lazy. I realize it won the Pulitzer Prize; that doesn't mean there are no weaknesses.
There are also, sadly, some of what i would call "personnel issues" for me in this show. I can't go into it in detail, for the sake of being discreet, but I admit I've lost quite a bit of my fire for the show do to these personal issues within the theatre.
Most things get better once everyone is off book, though, and as I said, I'm almost there. We have over a week without rehearsing the big moments in the play, because one person will be out of town, and the director doesn't like to have a rehearsal unless 100% of the cast is present. So, much of my work will be on my own until then.