With three successful records under his belt, John Mayer could pretty much do anything he wanted for his fourth album, 'Battle Studies.' So not only did the singer-songwriter move away from complex guitar lines in favor of a simpler sound, he decided -- on a whim -- to have Taylor Swift provide guest vocals on a song.

Mayer tells PopEater that the decision was as simple as "just my imagining hearing her voice in harmony with mine on this song that was sort of stemming up in my brain. It was a little bit like writing a screenplay with an actor in mind. I went to this place in my mind like Taylor and I were in the '70s playing on 'The Johnny Cash Show.'"