I've been reminding students this week about the fact that we need to process messages in order to acquire language. We have to comprehend, and in order to comprehend, we need to process. So this is an explicit question to students when I think I see fake responses or focus waning: "Are you processing what I'm saying?" If it needs to be said, I follow up with "If you're not processing, you're not acquiring." I try my best to direct this question to the entire class, to avoid shaming any one particular student, but I will make eye contact with those to whom it is directed.
Let me say first that I have a strong disdain for extrinsic motivators. Alfie Kohn (Punished by Rewards) opened my eyes to the pop behaviorism madness that exists in our schools and society. So to the extent that I can get away with, I don't bring up the "grade" aspect. It has to come up because I work in a school, but I try to limit its prominence or importance. Instead, I focus on the fact that one is wasting his time if not processing the language (because they're not getting anything out of it). I look at them and say, somewhat seriously, "Es triste." Everyone knows what that means. If they didn't, I'd say it in English too.
Some analogies to help this sink in for students who don't get why they need to process everything they see and hear in Spanish class:
-It's like going to the cafeteria and not eating, just watching everyone else eat.
-It's like going to the weight room and looking at the weights but not lifting them.
Of course my job is to make some real tasty food and serve it up fresh, so that they feel compelled to eat it. My job is to get some high-adrenaline music pumping from the gym speakers so that they can't help but pump some iron.
But I can't make them eat or lift. If they don't, es triste.