Tom Selleck, of course, is now the star of "Blue Bloods," a police drama that couldn't be further from "Friends" on a tonal level. But during a recent interview on "The Kelly Clarkson Show," Selleck revealed that he learned from his experience on the sitcom, and that he considers coming onto a well-established show as a guest star to be a particularly difficult gig.

"The hardest thing is to guest on a show," said Selleck. "I try to relax the new actors [on 'Blue Bloods'], because that's really hard, to come on a show where everybody is up to speed." Selleck's viewpoint makes sense, given how famously close the "Friends" ensemble was with one another, so it only figures that even a famous interloper like him would have a tough time fitting in. "That group is just an incredible group of friends. They obviously became friends in life as well as on the show and it shows," said Selleck. Despite admitting that he was intimidated, he said that Courteney Cox and the rest of the cast made the experience a positive one overall. "We ended up doing about nine of them," the actor said, underestimating his episode count by one, adding: "I was only supposed to do three!"