While he says everything he does in the studio is "very spontaneous," it sounds like Urban has actually been wanting to pen a song about his love for quite some time. He admitted, however, that dedicating a song to his significant other was difficult.
"When I create, rarely do I have a set idea," he explained. "'Gemini' started with the four of us in that room -- Ian Kirkpatrick, Justin Tanit and Julia Michaels and myself. Ian had a cool groove going and I started playing guitar and melody starts coming. Justin and Julia are asking me about Nic, 'Tell us about Nic.' And I said, 'Well, she's a Gemini,' and they're like, 'Ooh.' And Julia starts singing, 'Baby, she's both,' and the song just started to take off from there."
Asked to describe how he feels about Kidman, whom he's been married to since 2006, Urban told us that "the music speaks way more" than words ever could.
"You just have to feel it," he shared. "When you hear a song like that, or the flow of the record, it's all in there."
"I think she liked it," Urban joked. "I hope so. Yeah, it's a good dance song."
Urban also told ET that he worked on a lot of his ninth studio album from his home studio in Nashville, Tennessee, where Kidman and their two children, Sunday, 9, and Faith, 7, would often pop in. (Kidman also shares two kids, Isabella, 25, and Connor, 23, from her previous marriage to Tom Cruise.)
"I love it because it's -- most studios are like this, you know, no windows, you don't know if it's day or night. And I love lots of natural light," he explained. "The studio that we built has huge, big windows, so even on a gloomy day, it's pretty light in there. Lots and lots of natural light. I love working in the morning and it's just a really great environment to be in."
When asked if Sunday and Faith have any interest in Dad's work, Urban said "a little bit, yeah."
"Our 7-year-old has picked up the violin," he revealed. "[And a] bit of piano between the two of them."
If you listen closely to Graffiti U, you may hear a familiar voice on "Female." That would be Kidman, Urban previously confirmed, telling ET it was actually his idea to use her vocals, simply because "she was home."
"It was the perfect song, it was so right," he gushed. "She was home and I said, 'You need to come down if you can,' and, 'Can you sing on this?' So, she came down. I got great footage of her singing it in the mic."
When ET caught up with Kidman on the red carpet at the American Music Awards last November, she told us singing backing vocals didn't come easily to her... at all.
"I went down [to sing vocals] and I did it because [Keith] asked me to," the Big Little Lies star shared at the time. "He knows I have no confidence when it comes to singing. So I'll sing for him and that's it."
Hear more from the exclusive interview in the video below.