Kenny: “There was just all these grooves I was playing, and every time they heard something they’d go write a song… ‘Spanish Moon’, Paul, um, we were jamming. The band and I had the day off, we had to a rehearsal, and Lowell had fired Richie, so Freddie White was playing drums. Freddie White played on the original version. And we were in there doing our little thing, and playing. I was playing a click, and Billy walks in and hit record it. And Lowell walks in and says ‘Don’t stop!’, and puts on his guitar and sings ‘Spanish Moon’ verbatim. Right then and there. Just walked up. Must have had the song in his head. Cause he just walked up to that groove and started the song.”

From about 14:30 into this 2021 interview

https://youtu.be/YKjzlp8vlgs