The 'Radioactive' singer said he wasn't sure about the band's direction for their fifth album - which introduces instruments such as a fiddle and a trumpet to The Rock band's sound and features entirely ad-libbed lyrics from the frontman - until he plucked up the courage to play it to wife-to-be Lily Aldridge.
He explained: "I played her one song, and when I did, she looked at me and went, 'What the f**k? Why haven't you played me this? What are you scared of?'
"Before you know it, we had listened to every song, and she just loved it. That gave me the confidence to say, 'All right, well, maybe I'm in my head too much'."
On the subject of his spontaneous lyrics, Caleb - who is joined in the band by brothers Nathan Followill
He added: "I didn't write lyrics. I went in and ad-libbed, I free-floated everything. The closer it got to the end, I felt like, 'Man, you didn't do your job.'
"I kept thinking, 'When I go back and redo the lyrics, then I'll get it. But when I went back in to try to do that, everyone was like, 'What are you doing? You can't change those lyrics. Those are the lyrics.'"