A prank four years in the making.

Last Friday, late-night viewers were greeted by an epic April Fools’ Day prank when Jimmy Fallon and Jimmy Kimmel swapped shows.

READ MORE: Jimmy Fallon And Jimmy Kimmel Swap Shows In Epic April Fool’s Day Prank

On Monday, each host took fans behind the scenes of the epic switcheroo, which involved their swapping cities, staff, and networks to pull off.

In an interview with The Hollywood Reporter, the “Jimmy Kimmel Live!” and “The Tonight Show” hosts revealed that Kimmel had come up with the idea four years ago over dinner.

Testing out their video link in the behind-the-scenes footage, Kimmel remarked, “We are all, I have to say, very impressed and depressed at how nice your studio is compared to ours. How’s that s**thole in L.A. that you’re working in?”

In their interview with THR, the comedians talked about the most difficult aspects of pulling off the prank.

“Mostly, we wanted to make sure we kept it secret,” Kimmel said.

“I’m not really a prank guy, so I didn’t know if we could pull this off,” Fallon admitted. “Like, two seconds before they announced me, I was nervous. I was like, ‘I hope they like me.'”

READ MORE: Jimmy Kimmel, Stephen Colbert & More Late Night Hosts React To Will Smith & Chris Rock’s Oscars Altercation

Asked what it took to get the show to happen, Fallon said, “Tons of planning. Tons. And to keep it a secret. Our producers were talking to each other every day for the past month. But this probably took a year of planning.”

“We prepared more for this show than most,” Kimmel added. “What was most important was looking out for the other, sharing the little tricks you pick up over the years.”

Asked if they left anything special behind for each other, Fallon aid, “I left him a card in my office.”

Kimmel joked, “I left my youngest son.”