How do you teach self-motivation?

In a field like theatre, so much of learning seems to come down to self-discipline. It is not like math, where you either understand how to do the problem or you don't. It isn't even as tangible as music (where you either can or can not play the solo). And so I am left asking after many class periods, is there a way to motivate students to not slack?

For example, my acting class is currently working on Shakespearean scenes. And though they have all had their scenes chosen for at least two weeks, and though we have been working in class for the last three weeks on the text, still they come to class today to work on their scenes unmemorized, unblocked and frequently without the basics of motivation or intention decided. They perform the scenes for a grade on Thursday and I have no doubt that the work they do between now and Thursday will be at least equal to, if not more, than all of the work they have done up to this point.

I know that they all have busy lives, other classes to do work for and parts to memorize, but these are supposed to be the most devoted actors in the school... and if they won't work on a part more than two days out of the performance what am I supposed to do??? I'm dissappointed in their lack of work ethic but honestly, is that something I can teach? I supposed I could give them lots of little assignments along the way (turn in your objective for the scene... turn in your blocking for the scene, etc). But when it self-initiative supposed to kick in? If not in upper level college classes, then when?