(CNN) -- Rat, meet Spaceman Spiff.
It's not a collaboration Stephan Pastis imagined in his wildest imagination. And, as any reader of Pastis' comic strip "Pearls Before Swine" knows, Pastis has a pretty wild imagination.
But there was the proof, in black and white (and, in many newspapers and online, full color): "Pearls' " Rat and Pig sharing panels with the work of "Calvin and Hobbes" legend Bill Watterson, creator of the mischievous boy, his stuffed tiger and countless space- and time-defying alter egos.
For Pastis, a huge "Calvin and Hobbes" fan, the partnership was never less than intimidating.
"Just working with him, I was so scared at all points," he told CNN in a phone interview. "What if he thought, 'He's not as funny as I thought he was?' At every point, I thought he would go away, and what proof I would have that this ever happened?"
For Pastis, who usually works several months in advance, the strips came together remarkably quickly. He and Watterson first got in touch April 11, the day Pastis had a strip that falsely portrayed him as the author of "Calvin and Hobbes" in order to get a woman in bed. Some time earlier, he had been told by a mutual friend, Washington Post cartoonist Nick Galifianakis, that Watterson had been wanting to get in touch.
Watterson is a longtime "Pearls" fan, the "Calvin and Hobbes" cartoonist told The Washington Post.
"Several years ago, when Stephan did one of his strips that mocked his own drawing ability and mentioned my strip in comparison, I thought it might be funny for me to ghost 'Pearls' sometime, just to flip it all on its head," Watterson said.
Watterson and Pastis finally exchanged e-mails and Watterson suggested doing something together.
"I got that e-mail on a Friday. I think I literally stood up at the couch," Pastis recalls.
It may seem like an unlikely pairing.
Pastis regularly features himself as a character in his strip and enjoys touring the country, promoting it and his "Timmy Failure" series of children's books. Watterson is known as reclusive and private; there's just one known picture of him in existence. Pastis would be the first to tell you that he's not the world's greatest artist, and his characters -- Rat, Pig, Goat and Zebra -- are drawn with blunt simplicity. Watterson is one of the finest draftsmen to ever pick up a pencil.
And Watterson, Pastis learned, isn't the most tech-savvy artist. The "Pearls" cartoonist thought they could exchange work online, but Watterson wanted to use actual hand-delivered mail.
"I talked him out of that because I was so scared," says Pastis, noting that any Watterson drawings would be worth thousands.
But electronic collaboration had its own challenges.
"He could not set the scanner to get rid of the pencil lines. He had trouble even putting (his work) on the scanner straight. He had trouble with e-mail attachments," he says. But Pastis reveled in the trade-offs. "That part of it was funny. With anyone else it would be frustrating. With Bill, I cherished every e-mail."
Then came the matter of maintaining the secret. It wasn't easy, especially since Pastis was a finalist for the Reuben Award -- cartooning's highest honor -- this year. The group met in May for its annual honors, and it was all Pastis could do not to scream the news from the heavens.
"I'm in a room with 200 cartoonists. You know how hard it is not to say, 'Hey, Bill Watterson is drawing my strip'?" he says.
When Watterson's contributions started appearing on Wednesday, eagle-eyed readers may have recognized the style and lettering -- particularly in Thursday's strip, which was full of Martians and robots, "Calvin" favorites. But Pastis suspects many fans were thrown off by Friday's strip, which included two voluptuous women who looked like "Archie's" Betty and Veronica.
The reaction to the news has been tremendous, says Pastis. His blog and the "Pearls" comics page each did 10 times their normal traffic. It was enough to blow out a server at Universal Uclick, his syndicator.
Now that the Rat is out of the bag, the strips will be auctioned off. Indeed, a primary reason for Watterson's involvement was his activity with Team Cul de Sac, a charity co-founded by cartoonist Richard Thompson, who suffers from Parkinson's.
And Watterson may not be as reclusive as his image suggests. Besides his work with "Pearls" and Team Cul de Sac, he's also given an interview to Mental Floss magazine. He may not be returning to daily fare, but it's not like he's turned into a hermit.
In fact, after their long-distance collaboration -- which was done entirely through e-mail, without so much as even one phone call -- Pastis finally got to meet his hero last week in Washington when they went to visit Thompson. The pair spent hours talking about Charles Schulz -- a mutual influence -- the challenges of the comic strip business, and simply the wonders of humor. Watterson is "quick to laugh," says Pastis. "When you make him laugh, he really laughs."
And Pastis couldn't be happier.
"I'm still on Cloud 9," he says. "I will never top this."