A few weeks ago, after hearing a presentation by Melvyn Bragg, I made the point that effective broadcasters aren't necessarily as effective when it comes to public speaking (HERE).
I've also commented on how famous actors, with the notable exception of Ronald Reagan, aren't always particularly effective at making speeches either:
'After all, their skill is to deliver other people's lines in a way that portrays characters other than themselves, which is a very different business from writing your own lines and coming across as yourself.
'Politically active thespians like Glenda Jackson, M.P., and Vanessa Redgrave may be admired for their successful acting careers, but neither of them is particularly impressive when it comes to making political speeches.
'In fact, the only example of an actor who did become a great public speaker that I can think of is Ronald Reagan, but he'd already been rolling his own speeches on the lecture circuit for General Electric long before he became Governor of California...' (more HERE)