Eastwood, 82, told The Carmel Pine Cone newspaper in California he didn't clear what he was going to say with Republican presidential candidate Mitt Romney's staff before he took the stage in Tampa, Fla.
During an improvised speech praised by his supporters and mocked by his critics, Eastwood pretended to ask an empty chair representing Democrat President Barack Obama why he didn't keep his 2008 campaign promises.
"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,'" Eastwood told the newspaper, explaining he had the idea for the empty-chair interview minutes before he gave his speech.
The actor-writer-director, who was elected mayor of Carmel in 1986 and served one term, went on to say he wanted to make three main points during his RNC appearance.
"That not everybody in Hollywood is on the left, that Obama has broken a lot of the promises he made when he took office and that the people should feel free to get rid of any politician who's not doing a good job," he told the newspaper. "But I didn't make up my mind exactly what I was going to say until I said it."
Eastwood said Romney and his running mate Paul Ryan came backstage to thank him after his speech, showing no sign they were displeased with what he said.
"They were very enthusiastic and we were all laughing," he said.
"President Obama is the greatest hoax ever perpetrated on the American people," Eastwood said. "Romney and Ryan would do a much better job running the country, and that's what everybody needs to know. I may have irritated a lot of the lefties, but I was aiming for people in the middle."