The following is my response to a comment from Hillary on my recent on improving your message with my "Say It Badly" technique. I thought it was an interesting enough thought to deserve reposting.
The difference between speaking and writing is like the difference between improv and scripted comedy.
There is an immediacy and interactivity to improv that makes the experience compelling, even though the actual performance is much less polished.
The trick to speaking well is to let go the notion that speech should be as polished as the written word, and focus instead on emphasizing the immediacy and interactivity that make live such a impactful experience.