It is hard to lead a good discussion. I realize I don't always do it well myself in the heat of the moment, but I think I know something about how to do it.
Avoid yes / no questions. You want your students to talk, not just say yes or or no.
Don't be afraid of easy questions, especially early in the discussion. Ask for a character's name. How old do you think a character is? How do we know this? By the same token, avoid question with too many complicated parts to keep in one's mind. I find that students often lose track of the question if it is overly complicated.
Ask weak students easier questions to get them involved.
Move on if a question is not working. If you have to re-explain a question, substitute an easier (simplified) version of the same.
If you really want the students to discuss a particular thing, then you might have to ask similar questions that get at the question from different angles. After a while, though, it is time to move on to another topic.
Let several students try to answer the same question, or respond to one another. In a really great discussion the professor is often saying less and less.
Observe a student's body language and facial expressions. You can often tell if they have something to say. If students look disengaged you can call on them too, in order to bring them back (or make that attempt).
Rephrase the student's answer if you think other students have not understood. If you don't understand an answer, it is find to let the student try again. If the student gives a really good answer, you don't always have to say anything about it, if you think others have understood too. On the other hand, if a student has made a brilliant point and others have not heard, it might bear repeating. Once in a while the professor can summarize the general drift of the discussion in a succinct way.
The purpose of discussion is multiple. The professor is giving students a chance to contribute to the course, to teach one another, to practice their Spanish (or their oral skill in English or whatever language the course is given in). The professor is seeing how well the material is working with the class, how engaging it is, whether they are able to understand it. The students are learning how to hold their own in a discussion, how to relate to others in an academic setting. The class is bonding together as a group, creating a unique and memorable experience unlike any other class.