Few things are as deflating as pouring your soul into a talk, then having no questions at the end as your audience scrambles to escape. At some level, it might be your fault. Perhaps you left too little time for questions. Perhaps your demeanor did not welcome questions. Perhaps your audience was unfamiliar with your material.
If you are the host, however, you never want a speaker to leave your department and say there were no questions. So, always have a question in mind. Encourage others to do so as well. You might replace a question thought of early with a better later one, but always have a question in mind. You might not even need to ask it if there are enough others asking questions. The important point is that there should be questions. Thinking of what they should be can help you think critically about the material.
One of my colleagues at a different university once wrote a famous email to the department’s grad students exhorting the students to be more active in questioning and not to sit there like torpid dolts. I don’t know if it had the desired effect, but I do know that curious, questioning grad students can leave an excellent impression on a speaker.
There are a lot of issues surrounding questions. One is whether you should interrupt the talk with a question. If it is a job talk, or a very formal talk, the general rule seems to be to interrupt only if it is a point of clarification and you are fairly confident the answer will help everyone get back on topic.
Another issue is the balance of questions. I once gave a talk at a prestigious university that had a very troublesome postdoc who seemed intent on destroying all speakers. I was warned about calling on him before the talk. Even well-meaning people can sometimes dominate the questions.
Philosophers of science think carefully about just about everything, including how to handle discussion after a talk. Here at Wash U, the moderator sits in the front and watches for the hands to go up after a talk. He writes down all the names so everyone with their hand up gets a chance to ask their questions. They call on a grad student first. Then they take a long time as they carefully discuss issues. In the middle they ask again to be sure they got everyone who wants to speak on the list. But they keep it lively with a special trick, the different hand signal for a follow-up question.
I’m sure you have been frustrated in a discussion when everyone is on one topic for awhile, but some people bring up new issues while others go back to a previous question. The way the philosophers handle this is to have two different signals, the raised arm with an open palm for the new topic and the raised arm with a finger up from the hand for the follow-up question. That way one topic can be completed before the next one is taken up.
I like this. I also like a long series of questions and discussion. Ideally the talk would be shorter and the discussion longer, the way Lawrence Krauss ran it at Origins meetings. I don’t know if I can get biology to adopt the palm vs. finger approach to questions. We don’t generally have that much discussion, except at chalk talks.