When Castle premiered, it was unlike any show we had ever seen. It partnered a crime writer with a NYPD cop, who ended up falling in love. Richard Castle, the author, starts off as an arrogant celebrity who ends up becoming a romantic, goofy partner for Kate Beckett.

But there are some things even the best detectives (or biggest Richard Castle fans!) wouldn't know.

1. Couples Therapy

Their on-screen chemistry was always palpable, but in real life the actors hated each other. Stana Katic announced she was leaving the show after season 8, and that's when it was revealed that she and Nathan Fillion couldn't stand each other.

"Stana Katic and Nathan Fillion completely despise each other," said a source from the set. "They will not speak when they are off set, and this has been going on for seasons now. This season, it got so out of hand they made Stana and Nathan go to couples counseling together."

2. So Many References

Nathan Fillion, who played Castle, was also on a well-loved (but short-lived) television series called Firefly. The Joss Whedon show made many small appearances in Castle, with many of the actors taking guest roles. In one episode, Castle himself dresses up in his Firefly costume.

3. Say Again?

Stana Katic hid her (alleged) hate for Fillion pretty well, but that's not her only acting talent. Katic speaks fluent English, Italian, French and Serbian fluently. She can also break out perfect accents in English, British, South African, Greek and Spanish.

4. True Love!

Ryan and Jenny weren't just a cute couple on-screen, they were also married in real life! Seamus and Juliana Dever were married in 2006.

5. The Books Are Real

When you hear the characters talking about Nikki Heat or Derrick Storm novels, you can actually go and read them yourself! There have been seven Nikki Heat novels released, all of which placed on the New York Times Best Seller List. Additionally, there are two Derrick Storm books. Readers do not need to watch the series in order to enjoy the books, but there are a couple references to the show which viewers will get.

6. Ryan Is A Fan

In almost every episode, Detective Kevin Ryan can be seen carrying one of Richard Castle's books. There's usually a different one each episode.

7. Long Run

Nathan Fillion (Richard Castle), Jon Huertas (Javier Esposito) and Seamus Dever (Kevin Ryan) appear in all 173 episodes of the show. Stana Katic is credited in all 173, but she doesn't appear in two of those episodes. The three men are the only characters to appear in each episode.

8. O, Canada!

Both Stana Katic and Nathan Fillion are Canadians, which is rare for the leads on an American show. Fillion was born and raised in Edmonton, Alberta before moving to New York City in 1994. Katic was born in Hamilton, Ontario but spent her childhood moving between the U.S. and Canada.

9. Cut It Out!

When trying to cast Kate Beckett, Castle's creator and writer Andrew W. Marlowe says Nathan Fillion had to read with 125 different women. None of them were working out. In the end, it was down to Katic and another actress. When Katic showed up to read, she realized she didn't like the blouse she was wearing because it was too long. She grabbed a pair of scissors, and at the same time Nathan Fillion happened to spot her. He asked if she wanted to run lines, and she responded "No, but speaking of lines, can you cut a straight line?"  He replied "I can try." He cut her blouse, and the producers happened to be watching the whole thing. And that's how she got the part!

10. One Week's Notice

When it was announced that Stana Katic was leaving Castle after season 8, little did everyone know that would be the beginning of the end. One week before the season 8 finale, it was announced that the show would not be returning for a ninth season. ABC had cancelled the show, and now they were stuck. The series finale was considered an extreme let-down by most fans, as it wrapped up the entire show in about 20 seconds. Both Castle and Beckett are shot, left laying on the floor while bleeding out. Then, the camera pans to them being a happy, healthy family seven years later. A seven-year gap left fans wanting more, and wanting to know just what happened after the couple was shot.

Did you ever watch Castle?