"Castle" is now in the middle of season 8 and Nathan Fillion explained the longevity of the series, as well as revealing who among the celebrities he has a man-crush on.

"It's officially the longest I have ever done anything," he told Sydney Morning Herald. "What happens when you play a character for eight years? I have become invested in characters I have played for far shorter periods of time, so to say I am invested in Richard Castle is something of an understatement."

On why "Castle" survived eight seasons so far, Nathan Fillion said, "I like to think of it as being not tricked up. It's simple, real."

Another thing is that people can relate to it because it's based on reality.

"We met a lot of homicide detectives when we were filming the pilot in New York, and these guys were not dark, haunted, brooding, take-off-your-glasses, husky voiced guys. They were just regular guys," he said.

Meanwhile, speaking to Advocate, "Castle" season 8 star Nathan Fillion revealed his "man-crush," although technically he mentioned five.

"Umm, Benedict Cumberbatch, Jeremy Renner, Bradley Cooper," he said. "Who else do I think is really great? Both of the Wilson brothers. Who else? There was somebody I was just watching the other day and they were amazing..."

Nathan Fillion also credited "Castle," which is on its season 8, for bringing him to the mainstream, whereas "Firefly" generated a cult-like status among sci-fi fans.

"Castle is certainly a big deal to my career and the recognition factor. But I'm not out in the world much to experience that, unless I go to sci-fi conventions, at which point it's a toss-up between Castle and Firefly," the actor said.

"Firefly, having been canceled 13 years ago, is very strange in that it keeps getting more and more fans every year. Younger and younger people are coming up to me saying how much they love it, and I can't believe it. I'm so grateful for that experience."