What to do when intimidated? This is a tricky question that reaches far beyond just Aikido. when looking at some of the other responses it is clear that what you do is a very individualized decision.
For me I see intimidation tactics daily in what I do for a living and as such can't give into these tactics. Each case is different whereby some are best tackled head on and drawing the proverbial line in the sand for your aggressor never to cross. Other cases require a kid glove like approach. Regardless of the situation, it is normal to have your heart in your throat and be scared as heck, overcomming this is a victory in it's self.
At the end of the day dealing with intimidation requires that you have the where with all to know what your limits and or boundries are. Don't get in over your head.