Ask HN: How do I improve my impromptu speaking?
I struggle to gather my thoughts quickly when talking about random topics presented to me.
They also do a thing called table topics where at every meeting members are asked to stand up and speak on a random topic for two minutes. Sounds scary, but if you practice it regularly, you'll be surprised how quickly you improve.
For informative/thoughtful, you basically need to have coherent thoughts on a given subject before you speak. You can't just magically figure things out as you open your mouth, there isn't nearly enough time.
For entertaining effect, I guess it's more art than science. Try watching some good sitcoms and standup comedians perhaps?