Clint Eastwood said the idea for the most controversial convention speech in a generation came to him in the green room off stage, just after he greeted Archbishop Timothy M. Dolan, the country’s most influential Roman Catholic bishop.

“There was a stool there, and some fella kept asking me if I wanted to sit down,” Mr. Eastwood said. “When I saw the stool sitting there, it gave me the idea. I’ll just put the stool out there, and I’ll talk to Mr. Obama and ask him why he didn’t keep all of the promises he made to everybody.”

Mr. Eastwood broke his silence about what he acknowledged was a “very unorthodox” endorsement of Mitt Romney during the Republican National Convention in an interview with The Carmel Pine Cone, his hometown newspaper in California, published on Friday.

During Mr. Eastwood’s speech on Aug. 30, carried in prime time on the night that Mr. Romney accepted his party’s nomination for president, Mr. Eastwood engaged in a rambling and sarcastic conversation with a chair, telling the audience that an invisible President Obama was sitting in it. His talk drew tough reviews from political and media analysts on both sides of the aisle, who found its freewheeling style out of sync with the sober tradition of political conventions.

Mr. Eastwood said that the Romney campaign had asked for details about what he would say, but that he balked at the notion of divulging too much or rehearsing his speech.

“They vet most of the people, but I told them, ‘You can’t do that with me, because I don’t know what I’m going to say,’” Mr. Eastwood told The Pine Cone.

He explained to the newspaper why he spoke for about twice his allotted time. The Romney campaign had limited his remarks to about five minutes, he said, but “when people are applauding so much, it takes you 10 minutes to say five minutes’ worth.”

He said he was aware that his approach was unusual, but argued it was in keeping with the way he gave speeches as mayor of Carmel-by-the-Sea.

“They’ve got this crazy actor who’s 82 years old up there in a suit,” he said. “I was a mayor, and they’re probably thinking I know how to give a speech, but even when I was mayor I never gave speeches. I gave talks.”