One example of this was when I was representing KOTESOL at a conference in Russia and was asked to come to the front and say a few words about the conference that KOTESOL would be hosting the next year. No warning, no time to prepare, just get up there, smile, and wing it. This situation can even throw experienced presenters for a loop. No PowerPoint slides to fall back on, to time limits to base our amount of content on, just freestyling it in front a room of your peers.
The biggest problem that occurs in this situation is that we revert back to all out bad public speaking habits. We forget to make strong eye contact with different parts of the room. We lose track of what our hands are doing. We forget that most audiences want to hear less, not more, so we ramble on and on with no organization to our thoughts.
It's good to give students the opportunity to experience what it's like to be called on to speak in front of a group without being able to sufficiently prepare. I do this with my students in three stages.
In groups of three or four students, I give them a personal topic such as their hobby or favorite restaurant (listed on a PowerPoint slide) and give them three minutes to prepare and then one minute each to speak. I use the timer on my phone to keep track of the time and the beeping means the time is up. The students can stay for this stage seated but they need to adjust their chair so that they can easily see all of their group members. All of the group members talk about their experience with the same topic.
This time I give them a new topic and only give them one minute to prepare. The talking time is 45 seconds. I usually have them stand up for this stage. Be sure to start with a different group member this time.
This time they will not be given any prep time and the speaking time is 30 seconds. The twist is that when it's time for the second presenter, I change the topic. For this round, every student has a new topic. *evil laughter*
It's important to explain to your students the difference between high-stakes and low-stake presentations. They won't lose their jobs if they mess up in your class. Your classroom needs to be a safe space where students view a bad presentation as a learning experience and not something that will damage their reputation or harm their grade (when it's for practice). If you can get your students to relax and look at the class as an opportunity and not a threat, this activity will be both practical and empowering for your students. Best of all, it can be done in less than thirty minutes. Good luck!