So you can do
say -v Samantha Help, I'm trapped in this castle!
or say -v Princess Help, I'm trapped in this castle!
But wait, it gets even better: The Mac comes with a number of voices for other languages, such as French, Hebrew, Chinese, and Arabic. And it turns out that the voices will work in English as well, using that country's accent. I just tried it with a few different languages, and was amazed by how well this worked, within reason: say -v Sin-ji "Hello, I am Chinese. What do you want to eat?"
say -v Thomas "Hello, I am French. What do you want to eat?"
say -v Carmit "Hello, I am Israeli. What do you want to eat?"
And yes, I should get back to work...