Picture yourself on the White House lawn. It is a starry evening; the glasses are clinking; the exact date and year are uncertain. The chief of protocol is overheard introducing the guests. “Abraham Lincoln, have you met Henry Fonda?” ... “Anthony Hopkins, do let me introduce you to Richard Millhouse Nixon; I almost feel you are old friends ... ”

It’s only a dream. Or perhaps a nightmare. But doesn’t it spotlight the bizarreness of a screen tradition that vaults space, time and reality yet has flourished happily for a hundred years? And who would bet against another meeting between a statesman and his ideal impersonator a year or two hence? “Barack Obama, let me introduce Denzel Washington ... ”