The 12th precinct takes a trip back in time this week in Castle Season 6 Episode 20, as our favorite detectives get down and get groovy in order to play into a witness' delusion that it's still the 1970s.

We chatted with Seamus Dever to talk about his love of all things '70s, what he'd like to see in Kevin Ryan's future and what fans can expect from the upcoming Castle season finale. 

Castle Photos from "That '70s Show"

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How did you prepare for this episode? Were you channeling your inner Starsky & Hutch? 

I think, you know, Starsky & Hutch they were a jumping off point for the start of the character but I'm a big fan of movies from the '70s so it wasn't too hard to channel. There were some other influences. Some blaxploitation influences for sure. Starsky & Hutch, they were a starting point but it really got bigger than that. 

You sound like a big fan of the '70s. What was your favorite '70s TV show or movie?

Oh my God, yeah. There's so many movies. I mean all of my favorite movies are from the '70s. The French Connection, that sort of started it off. There's some really good film making. Chinatown, it's about the '40s but was made in the '70s. Super Fly, Shaft. Some good, just like gritty movies.  Billy Jack. Oh God, that was so like rough. It's fun to watch those movies where everything wasn't necessarily so polished. Most of my favorite music is from the '70s, I have to say, all like the soul music. We were definitely getting down with some Isaac Hayes, with some Bill Withers. It was a lot of fun.

Were there any '70s consultants on the set or did you all just dive into on your own?

Who needed them? I've got a pretty good knowledge of the '70s. I was born in 1976 after all so I consider myself a '70s expert.

Out of the entire cast, who was the most into this concept? Who had the most fun with it?

Oh boy. I don't even know that anybody was having any more fun than our crew was, to be honest. We all had a lot of fun and there were a lot of laughs. Penny (Johnson Jerald, Capt. Gates) had a good time. Tamala (Jones, Dr. Lanie Parish) had a good time.

Jon (Huertas, Det. Javier Esposito) and I had a really good time. Yeah, it was a lot of fun. Nathan and Stana they got a kick out of wearing the clothes and seeing us dressed up so different. Sort of the general consensus was that Jon and I were the most transformed. I think when you see the episode it will be evident. But the crew had a good time because of it. They got to do hair and makeup on us and make us look so different. Kudos to them.

Your costume designer Luke Reichle did an amazing job with your outfits but were the actors allowed any input?

Well, of course. In all of the fittings we do. We sit down, or really stand up and we put on the clothes and say this works or this doesn't. This is too tight, this is whatever. From that moment on, we were sort of like this is going to work or I like this. There's sort of a little spark that goes off when you put on the right pair of pants for the character or the right shirt and you go, oh my God. Let's take a picture and let's show everybody. Yeah, Luke had a great time. Him and Johnny, our other costumer, they got to pull some stuff together. They're all costumes.

They're all authentic. Nothing was made for this. It's not hard to find these in these great, big costume warehouses that are all throughout Los Angeles. Western Costume, they all exist, these costumes and they're authentic. You're wearing some acrylic turtleneck from 1977 and you know, it smells like it's been around since 1977. And you're just like, please don't rip. Please don't rip.

Is there anything from that time period you'd like to see back in style?

Soul music. I'd bring back soul music and it would be the only music you'd hear on the radio. Just so much music from that era that I absolutely love. My God, the O'Jays, the Dramatics, the Chi-Lites. Yeah. I listen to a lot of that kind of radio on my SiriusXM, Soul Town. It's my favorite. 

Now I've got to ask, do you have a favorite '70s song?

There's so many. Probably the one I'm into lately is What You See Is What You Get and I believe that's by the Dramatics. So, yeah. There's some good music out there.

Changing gears a bit, if you could pitch one idea for Kevin Ryan's story going forward, what would it be?

Oh, for Ryan. Geez. OK, two things. One, I want Ryan to go undercover again. That was a lot of fun. I'd want there to be one more secret that Ryan gets to surprise people with, just like we did with "The Wild Rover" episode in season 5. One more thing to make people go really, he did that? I want that moment just one more time. And then the other one I've been pitching for years that I don't think is ever going to see its way in there is a murder mystery where we all go out of town to some mansion in the Hamptons and they have some sort of stay overnight Clue kind of thing.

So I've wanted like a Clue episode, where we have to dress as these characters to flush out the killer. I've been pitching that for three years so I don't think it's going to happen. If it hasn't happened by now, but I think it would be really fun to do that. 

Will we see/hear more about little baby Ryan soon? It's been pretty quiet since she was born. Will we get anymore insights this season into the Ryan family?

No, probably not this year. Probably next year they'll pick that up but its not planned for this season.

We all know there's a wedding approaching and everyone is wondering, will Ryan be Castle's best man?

There's a fun episode after the '70s episode. I think it's Castle Season 6 Episode 21 where Esposito and Ryan both decide that they're worthy of being Castle's best man. So they're both competing for that honor. And Castle has no idea why they're acting like such suck-ups all of a sudden to curry his favor. Beckett's on to it very early and kind of lets the lunacy go on because she enjoys it but Castle's totally unaware of it and thinks that we're talking about something else. So the entire time we're giving him gifts and doing all of these things and just being like really affectionate, like you're my best buddy. We're friends, like best friends. Jon's really funny in that episode. He does some really understated throw away lines that are just really hilarious. Yeah, there's a competition and at the end you'll find out who the best man is. 

What can you share about the season finale? Will fans be happy or angst ridden going into the summer?

I think they're…um. Let's say, oh boy. Let's say they're going to want to tune in in September to find out what happens next. They're going to be challenged. I don't think they're going to be happy or sad. They're going to be challenged. Which is good. We want people to come back next year.

C. Orlando was a TV Fanatic Staff Writer. Follow her on Twitter.