To mark the show's return, Digital Spy got on the phone to series star Katic - who was on her way to set to shoot the crime drama's 2012 Christmas special - to chat about the different sides of her character, working with cult icon Fillion and what she wants to see in Beckett's future.
It can be tricky for a crime procedural to stay fresh but Castle seems to have cracked it - are you happy that the show keeps evolving and changing?
"I am, I think it's a new show now. I think season five is almost like our second stab at season one. I think we're more than just a standard procedural, so that's why we have that opportunity and latitude."
You and Nathan have been working together for five years. Is a lot of performing on the show second nature to you now, or are you always trying to develop that relationship and keep it fresh?
"I think each actor is creative in their own way and we are both creative in our own ways and I think because of that it stays fresh. Also, I think we are in a different stage with these characters now so we get to explore different types of stories at this point.
"There's some elements at this stage of the game where it's by rote - not by rote in that it's boring but just that we know the deal, we know how to play off of each other... but there's a lot of stuff that's new, so it's a constant exploration."
How important was it that Castle always had a personal slant, as well as the crime stories?
"I have to believe in the character to perform it. I think that's been a constant dialogue with Andrew Marlowe, the show's creator. How do we keep pushing this character into a space where other women in power find themselves?
"There are a lot of elements to every woman that's in power and one of them is that she's vulnerable, another one is that she's strong... and sensitive and sensual. There's all the colours and I think that it is interesting and true to form to play with all of them."
Speaking of different colours, Castle is known for balancing drama and humour - but do you think there's a limit to how dark the show can go?
"I don't know that we've gone so dark that we couldn't go back. I think that's one of the wonderful things about a story like that. I don't know if you've seen Argo or End of Watch recently? Those are two films I've just watched this past weekend and I thought they were really well told, and part of what was so enjoyable about them was that they told the dramatic tale, but also gave us a moment of humour. The moment of relief came through the humour, and I think that's something that Castle offers its audience."
Is it fun getting to break formula occasionally and shoot something like the film noir episode 'The Blue Butterfly' from season four?
"Oh my goodness yeah, absolutely. It's great fun to explore. We have really talented crew members - costumes, make-up artists, the camera department - the list goes on. I think that everyone loves the chance to really get down and dirty with their talents.
"Those [special episodes] are chances for us to play in those departments, and of course we recently shot an episode that was like The Office or a documentary-style series. That's again another opportunity to play in a different format, and this show kind of gives us that space to jump around in different ways."
You have a Christmas episode this year - have you shot that yet?
"We're in it right now; I'm just on my way to work."
How's it been shooting that episode?
"It's very funny. We've put Santa Claus in a lot of compromising situations!"
Castle has a huge following. How would you describe your experience with the show's fans?
"Well, first of all I'm very grateful. I think the reason the show sustained - at least for its first two years - was because of a wonderful grassroots following that I think found a big voice on the internet. Twitter became one of those vehicles for a lot of these fans, and I'm very grateful to them.
"In general, we have a really sweet group of people that enjoy and follow the show. It's not an experience that is just US-based - I mean, it's really a global fanbase, which is quite exciting and seems to be bringing a lot of people together all over the planet."
You won a Prism award for your performance in season four episode 'Killshot'. As an actress, how much do awards matter?
"They're very nice. I don't look to them to define anything or to define any performance. Of course, it's sweet to be acknowledged in some format, but that's not the central purpose behind doing what I do.
"I just enjoy working with really wonderful actors and amazing creative people and I hope to keep doing that, no matter where."
You're currently in your fifth season and when we spoke to Nathan, he suggested that six or seven seasons would be an ideal run for the show. Do you have any thoughts on that?
"I don't really know. I think as long as we're exploring the characters and we keep growing the story... that's the only request that I would have. I think that we are reconfiguring so much of the show and redefining a lot of it and exploring new territories, so we'll see. It's in the hands of Andrew Marlowe, it's in the hands of his writing staff, and Rob Bowman and his directing staff, at the end of the day."
Castle returns to Channel 5 this Friday (November 9) at 10pm.