Public figures are different than other speakers.
Somebody who speaks for a living -- whether they're the local news anchor, a professor who lectures all day or a motivational speaker -- is usually a specialist. They don't need to cross-train with different types of speeches.
In public relations, you can't rely on being good at one type of public speaking. One day might be packed with at press conference in the morning, a television interview at noon and a debate at 7 p.m. The next day might have a keynote speech, a panel and a radio appearance.
Cross-training is crucial.
I bet you've watched a famous movie actor host the Oscars show and bomb, or seen a them do a press conference where they seemed nothing like the smooth and confident superhuman they are on the big screen.
It's because being really good at movies -- where you memorize lines and get two dozen takes to do it right -- is nothing like performing live or getting interviewed by a reporter.
This is why the Oscars have turned to stand-up comics as hosts time and time again.
Stars with roots in stand-up comedy -- Ellen Degeneres, Jamie Foxx, Steve Carrell, Jay Leno, Ben Stiller, Steve Martin, Mike Myers -- excel at hosting shows, doing radio shows, getting interviewed and pretty much anything you can throw at them. It's because they're used to being up on stage with no script, no notes and no net.
So if you want to become great at giving big keynote speeches, don't just practice giving 30-minute speeches. Give impromptu speeches without any notes at all. Do debates, panels and forums.
The more you cross-train, the better you're prepared to speak at any occasion.