Baio, best known for his role as Chachi Arcola, now plays a former television star turned stay-at-home dad on the same Paramount stage where they used to film Happy Days. He cast Marshall, now 78, as his onscreen father.
"I went, 'Wow, I don't know if he would do it, but I'd love have him on the show,'" Baio told The Hollywood Reporter. "I called up his office and asked him, and he said OK. And we thought it would be a great idea for his son Scott to direct the episode, and it all worked out."
"I am here because of him," Baio said of Marshall, who's directed films like Pretty Woman and The Princess Diaries. "He taught me everything I know about television, and I owe him everything, so it's just one of the most gratifying things I've ever done."
"It was the same old stage, and so everything was familiar. I used to watch my son run around backstage, and now he's in front of me directing me and yelling at me to move over," Marshall said.
Marshall has guest starred on other series, including Louie, ER and Brothers & Sisters. He's also slated for an upcoming appearance on Two and a Half Men.
Baio and Marshall's reunion episode airs this Sunday at 8 pm on Nickelodeon.