HOWEVER, I played John, the witch boy many years ago in the play DARK OF THE MOON.  We had a VERY LONG rehearsal schedule, and I found myself in that same unenthusiastic predicament.  Everything seemed stale and disengenuous.  There was a scene I had with a trio of witches and we all got in that same rut, so we agreed to 'stir it up' a bit.  Before every performance we would decide which ONE of us would change a line or two slightly, always making sure the content of the line was the same and the cue line was unchanged.  We all knew the lines so well it was easy to 'improvise' and nobody ever got 'blindsided.'  It also made us 'think on our feet' and react to each other fresh anew.  Now, I would NEVER suggest you or anybody do the same (hey, we were young and looking for a boost), and I'm sure we gave the director royal fits, but that scene was always fun and something I looked forward to.  By the way, I was awarded 'best male actor in a leading role' for that part, so it wasn't just my imagination that what we did worked.  And yes, that could very well have been the exception that proves the rule.