This practice is not new to Clint Eastwood. The Smithsonian reports:
As part of yesterday’s showings at the Republican National Convention, famed actor and director Clint Eastwood startled and amused viewers by mock-debating an empty chair, meant to represent President Obama.
Many who saw the scene thought it to be strange and bizarre, let alone unconventional, for a forum that is usually meticulously directed. Delegates on the convention floor, however, loved it.
But it turns out that the history of debating empty chairs is a rich one, stretching back to at least 1924 when Progressive* vice-presidential nominee Burton K. Wheeler took a stab at an invisible President Calvin Coolidge.
Safire’s Political Dictionary describes the event, quoting from Wheeler’s autobiography Yankee From The West.
In Des Moines, I hit on an original showmanship gimmick. The hall was jammed to the rafters… I said, “You people have a right to know how a candidate for President stands on issues, and so far President Coolidge has not told you where he stands on anything… so I am going to call him before you tonight and ask him to take this chair and tell me where he stands.” People in the auditorium began to crane their necks to see if Coolidge really was somewhere on the premises. I pulled a vacant chair and addressed it as though it had an occupant. “President Coolidge,” I began, “tell us where you stand on Prohibition.” I went on with rhetorical questions in this vein, pausing after each for a short period. Then I wound up: “There, my friends, is the usual silence that emanates from the White House.” The crowd roared in appreciation.
Read the rest here. And in case you did not see Eastwood in action: