This is so double rainbow. They sang together in Dr. Horrible’s Sing-Along Blog—Joss Whedon’s masterful answer to the 2008 writers strike—and now Nathan Fillion wants Neil Patrick Harris on Castle.

So much so that at Comic-Con yesterday, Castle boss Andrew Marlowe publicly called out Nathan’s Dr. Horrible costar and deemed it: The Neil Patrick Harris Challenge.

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“I think he’s scared to come on the show because of your level of talent,” Andrew said to his leading man, Nathan. “I challenge Neil to come on the show. We will write a role for him.”

So what does Nathan Fillion think? As a fellow actor, Nathan knows that Neil’s schedule might keep him from guest starring on the show. And he respects him for it. Or something like that.

“The only thing holding Neil back, really, is his talent. He’s got so much that, first of all, he’s full of himself. And he knows it,” Nathan deadpanned when talking about his Dr. Horrible costar. “But it keeps him really, really busy. You’ll call him up and say, ‘Hey, I was just calling to see if maybe you were interested in doing an episode,’ and he goes, ‘Oh, I can’t stop now. I’ve got to rehearse for the Oscars.’”

Oscars or not, the challenge has been issued and Nathan is already planning that episode out. “I think it would be cool if we could kill him,” he joked.

Here’s hoping that Neil catches wind of this throwdown and starts rearranging his schedule!