There always comes a time when you have to give a speech, are playing a concert, are live on T.V., I don't know, a bunch of situations where you can have some situation or run out of things to say, maybe you're waiting for someone, so, what do you do? Improvise.
The first thing you'll want to do is not stray away from the topic. If you do, you'll end up doing something ridiculous. I saw somebody start a speech about the sexuality and ended up talking about music. A speech doesn't work like that. Also, don't stop talking. If your speech had an awkward silence that lasted more than ten seconds, it was a failure.
Now, if you're giving a concert, and the rest of the band had to go rest, or somebody got hurt, you're going to have to play a bit, something you really know how to play, or make everybody sing a song. A really easy one's "We Will Rock You". Of course learn the lyrics. If you play guitar, make an awesome solo. You can drum solo, or bass solo. Any solo will work.
I don't know much about television, but I do know this. Don't do anything you don't know about. You don't want to apply a Juay De Rito out there, so don't do anything stupid.
That's actually kind of it. Just do whatever that's in the top of your mind, and do it well. If you panic, you will fail miserably. If you succeed, you'll do it great. So, have fun with it, and don't blow it.