The awkwardness stems from a 2015 interview, during which Kidman told Fallon that she'd been open to dating him back when she was single. Fallon was left flustered by the "unbelievable" reveal, which Kidman later told ET she regretted.
During their Tuesday chat, things continued in the same vein right off the top, when Kidman, who was appearing virtually, shared her disappointment at not being in-person at the New York studio.
"New York's a special place for you. You got engaged here," Fallon told the actress, who tied the knot with Keith Urban in 2006.
"You're not meant to know that," Kidman replied, to which Fallon responded, "I'm allowed to know all the stuff. I know all sorts of stuff about you."
The interview moved to her courtship with Urban next, with Kidman recalling the moment she fell for her husband as Fallon congratulated her on 15 years of marriage.
"For a second there I thought you said 50 years," Kidman said after Fallon's anniversary statement. "And I'm like, 'No!' Hopefully one day."
The host went on to inquire if Kidman or Urban had pulled off a surprise for their milestone anniversary, before remembering that the Nine Perfect Strangersactress has a distaste for surprises.
"Every now and then I like a surprise, but it has to be a small one," Fallon agreed. "I feel like I'm tricking someone or something. I don't like to do that. I feel weird. I don't like surprises. I like to know what's happening. I'll act surprised."
"Yes, we know that," Kidman answered, seemingly alluding to his apparent shock at her 2015 dating reveal. As Fallon fanned himself and wiped sweat off his face, Kidman quipped, "Did I just burn you?"
In an attempt to get the interview back on track, Fallon wondered if Kidman and Urban would ever release music together. Kidman was quick to shoot the idea down, before praising her husband for his epic performances.
"He's playing Vegas right now, which is why I couldn't be with you, because I chose him. I mean, in terms of the Vegas [thing], to be here," she said with an awkward laugh, before chastising herself, "Oh, shut up, Nicole."
Fallon dissolved into a fit of laughter at that point, lamenting, "I can't ever right this, I swear. I can't right this. What are we doing?"
"What are we doing?" Kidman asked through laughter of her own. "Ask me something good! Sorry."
"I am. I will! Just stop it. Just chill out. Just calm down for two seconds. Gosh," Fallon answered, before concluding the interview with talk of Kidman's Hulu series.
As Kidman mentioned during her Fallon interview, she's been spending time in Vegas with Urban amid his Caesars Palace residency, Keith Urban Live. When ET spoke with the 53-year-old country star, he gushed over his wife being a "life-changing" influence in his life.
Watch the video below for more of ET's interview with Urban.