I can't tell you why he can't tell Joseph because: 

A) Joseph would find a way in his breaking to stop him most likely(Or so I am guessing from the way you've played him) 
B) It would indeed give away a huge portion of the plan
C) The scene in my head is worth it and it will only work if Joseph doesn't know until the right moment *Grin*

Posted! Alexi likes monologueing, and if you want me to change the part with the book or the water bottle, just let me know.