Maybe you aren't struggling with this, but I am and a lot of teachers I talk to are as well.
We are dealing with a different generation of students in our classes nowadays; as I have blogged about before, they are less kinesthetic in class, more self-advocating, and very, very relationship oriented. With you and with each other. It's all about who "likes" or doesn't "like" them. I believe that we've got to approach them from a relationship-oriented mindset or we won't get far in teaching them.
I've chosen to stop trying to change their basic psychological makeup and rather, to try to understand their reality and work within those parameters to teach Spanish as best I can.
I get student actors mostly by having students choose other students to be in the skit, not by asking for volunteers. Kids hate volunteering to act nowadays because for this generation it is generally not cool to go up there on purpose and draw attention to yourself. We need to understand that about this group of kids and just go with it. If you notice, they will act (usually) if it’s not their choice; either you (the teacher) “makes” them act, or you are drawing names from a hat, or in the case of the Mad Lib scripts I've written for the second half of 3A, the actors were put in the skit by the class decision when no one knew exactly what they were going to have to do in the skit.
Secretly, your most unenthusiastic, unsmiling actors are actually enjoying the attention of being in the story; they just won't show it for a million dollars because that is not cool. Know their secrets, Daniel-san. Work with them. Do not fight a battle you cannot win and lose your basic focus--which is providing comprehensible input in the target language in a low-anxiety situation.