It's not every day an actor gets to step into the shoes of immortal beings, let alone those at the center of a hugely popular fantasy novel. David Tennant and Michael Sheen, genre immortals themselves, will do just that as the stars in the upcoming TV adaptation of Neil Gaiman and Terry Pratchett's Good Omens. Tennant will play Crowley, the demon who happened to have delivered a certain apple to a certain woman in a certain garden, and Sheen is the angelic Aziraphale.

Despite playing for different teams, the pair end up being friends and have to team up together to stop the end of days, which would have a pretty substantial impact on their earthly habitats.

Sitting down with the two actors at the SXSX Film Festival resulted in a really fun conversation about how these two genre heavyweights built their characters under the watchful gaze of the original source material's co-creator. They both have lots to say about their time making this series, so let's get right to it, shall we?

I love that you have two very opposite personalities, an angel and a demon, that are friends. Can you talk about how rich that is for you as actors to get to play a relationship like that?

Sheen: It's such a crazy world that the book creates and there's such amazing fantastical, ridiculous, huge things going on that these characters, I think, kind of anchor it for an audience, don't they? It's what allows you to have an in, even though they are a demon and an angel. You connect to that kind of odd couple sort of dynamic between them, don't you? So that's definitely the way in for the audience and I think was our way in as actors.

Tennant: Without a doubt. I didn't know the book, unlike Michael. The script was my first experience of it and therefore the scenes, which were a lot of the time, just Aziraphale and Crowley talking and figuring it out. That was just delicious from an acting point of view. And also as an audience member, because that first time you read the script is the closest you'll ever know to what an audience will receive for the first time. That was what put me in, this relationship, these two characters and the fact that they were extraordinary and supernatural and yet at the same time very human and very relatable, actually. Even though one's a demon and one's an angel they're neither good nor bad. They're very much sort of muddling through like we all do.

Sheen: The book has such a particular voice and color to it, doesn't it? It has a kind of a tone to it that is very unique and I think part of what makes people love the book so much. Making it into a TV series, one of the difficult things for us was to work out what is the tone of this? How do we bring that same tone from the book into the TV show? How do you do that?

I think one of the big things about that was that we just anchored ourselves in this relationship and thought "Well, the tone will have to take care of itself around us." We just make that (relationship) kind of real and believable.

Then the other side, the other bit of help, was that Neil was at the heart of it. He was the showrunner, he's done the adaptation himself, he was there every day, he was the person that we could go to to ask questions about whatever. He just anchored that. That gives you a lot of confidence.

Tennant: Especially when the material is it has been around in the world for a long time and is just loved and beloved by so many people.

Video of Good Omens - Official Trailer | Prime Video

And so many people who have been waiting for so long to see the book brought to life in this way.

Tennant: Yes. People love this book. To have Neil there as our sort of cover in a way; one of the voices of the original novel is telling us we're getting right, so if you don't like it... (laughs)

Sheen: I come at it slightly differently. I read the book when it first came out. I was in drama school at the time. I had just very recently been introduced to Neil's work by a friend of mine at drama school who said, "Here. You need to educate yourself on comic books." He gave me Watchmen, Sandman, V for Vendetta, Swamp Thing, Hellblazer. And Sandman was the one I really connected to the most, I think.

So I knew of Neil's work through that. When this book came out and he was one of the co-writers I read it, loved it and so I'm a fan of the book as much as the fans who are out there waiting to see this come out. On the one hand, I kind of get why people love it, how much it means to people, how protective people are of it and the expectation around getting it right. And on the other hand, I'm one of the people who's part of the team trying to get it right. Having Neil there really helps that.

I think he was very smart in that on day one of shooting he took a photograph of me and David and just put it out on social media immediately. So people reacted then and there to us. Like, "not as tall as I imagined," or "that's exactly how I imagined it." It worked out quite well in that we realized that we were being accepted as these two characters. So that took a lot of weight off. It could've gone the other way. If people are gone "this is not at all what it is supposed to be like! They got it completely wrong!" That would have been an uncomfortable shoot.

It is fairly uncommon to have the creator of the material be there in that capacity.

Sheen: Often times by necessity you have to do things differently. Just because it works in one medium that does not in any way mean that it's going to work in a different one.

Tennant: And it's not every novelist who would be able to actually understand that, I think, and be receptive to the collaborative process of making something into moving pictures.

You're right that some novelists get that more than others. I mean, if Stephen King had his way we never would've gotten Stanley Kubrick's The Shining.

Sheen: Yeah, yeah. There are big, bold things that Neil has done at times with this that are different from the book. I mean the first half of episode three is completely new. We see the relationship between Crowley and Aziraphale developing over the millennia since the Garden of Eden. Now that's bold storytelling, but that's also understanding what works really well in the medium of TV as opposed to the novel. The fact that Neil had the confidence to be able to do that and to keep that strange, unique, eccentric tone that the book has, it would have been hard to trust that someone else could have done that, had it not been Neil.

Going into this project, was there anything that stood out as particularly challenging, whether it was a specific scene or a relationship or a tone to strike?

Tennant: It was one of those jobs where every day was rather delicious. There was something to look forward to almost every day because it spans all of eternity. There were very few days we went, "Oh, it's another day doing that kind of a scene." Everything had a very distinct feel and every day had a set of challenges that were more fun than the last. It was a bit of a whirlwind, this shoot. It was a long shoot and yeah, it, it kept being novel and surprising and it was one of the best jobs, frankly.

Sheen: I think the biggest challenge, probably for everyone, was how do you take something that is so enjoyable in its absurdity, and the asides are so enjoyable with Good Omens; it's all the stuff that's going on around the periphery. How do you get your audience to care about the narrative drive of the story? Because that's what will hook people. So, how do you find that balance of getting people to care about what's going on and then at the same time being able to enjoy all the stuff that comes along with that?

I think for us that was very much anchored in we have to get them to care about us as characters and this relationship and the stakes of that. That's the way into that. That was the challenge, playing such characters in quotation marks, how would you keep them real and believable in terms of the dynamic that's going on between them?

Tennant: Especially when they can get dropped into ever more extraordinary situations throughout history. But yes, I think it just keeps coming back towards that relationship. Where is it at now? How are they coping with each other and how did their relationship with each other forward the story?

Video of Good Omens Cast On Franchises And Fandom | NYCC 2018 | SYFY WIRE

Is that relationship something you guys had to do anything special to build or was that all there in the material?

Tennant: I think it is there in the material, so then you're just trying to be worthy of the material, really, and not let it down. All too often with jobs the first bit of the morning is figuring out how are we going to make this script actually make sense. "If I take this bit and you take that bit..." There was none of that. It was quality stuff to start with, so then it was just kind of playing, really.

Sheen: I mean, we weren't improvising or anything.

Tennant: No. Well, there is one bit of improvisation which made it into Episode 1.

Sheen: That's true! And it's Neil's favorite line!

Tennant: (laughs) Yeah. Again, you didn't feel the need or the desire to improv, you just want to service these scenes as they were written.

Sheen: Yeah. It wasn't like we were making stuff up necessarily, but having said that, it felt that we had enormous freedom in terms of how we portrayed the characters and how we related to each other and that kind of stuff. That didn't feel prescribed at all. That was there to be explored by us, I think.

One of the things that is very similar about us as actors, David and myself, is that I think we're quite inventive actors and we bring a lot to the table in a scene. You know, we're not just coming in and saying the lines. There's a lot of invention going on. We've thought through the characters and the scene and it felt like there was a lot of freedom for that. We were cast for that, presumably, and we hopefully delivered that.

It feels very inventive. There's nothing worse, I think, than for actors who do like to be inventive to feel shut down. That was not the case at all. So that felt very liberating, but when you've got really good quality material you don't have to make stuff.

Yeah and it sounds like I'm kissing your ass, but you both have great range? A lot of actors would be afraid to do a performance like you did in Tron: Legacy, for instance. I have to imagine that gives Neil a lot to work with knowing you both can go big and make it believable.

Sheen: I think we both have, at different times, played characters and in genres that can be quite fantastical and heightened. But I think part of what we both relish as a challenge is how would you root and ground that two-way stretch that you've got going on.

Tennant: Yes, and if you can hit that sweet spot of being entirely real and entirely grounded and at the same time having as much fun as possible. It's stretching that elastic, isn't it?

Even the most serious of Shakespearean actors play those roles with a twinkle in their eye.

Sheen: Absolutely. It's entertainment. We did a scene in this in Shakespeare's Globe at the original production of Hamlet! But it has to be entertaining. Shakespeare was writing big entertainment for final common people. There were ghosts and there were witches and there was fighting and there were swords!