The first thing to know is that famous people are usually very ordinary. They often (although not always) have a single marketable talent, are mostly self-obsessed and, probably as a consequence of their sheltered lives, they are largely ill-informed.
There are, of course, celebrities who can speak intelligently about something other than themselves. It’s been my privilege to have had some fascinating conversations with Live Aid organiser Bob Geldof about the craft of journalism, Glenn Frey of The Eagles about parenthood, left-leaning singer-songwriter Billy Bragg about pop and politics, and a pre-meltdown Mel Gibson about Shakespeare.