In the “Book of the Damned” episode of The CW series Supernatural, Charlie (Felicia Day) is back and calls Sam (Jared Padalecki) and Dean (Jensen Ackles) to tell them she found the book that could help remove the Mark of Cain. But of course, it wouldn’t be great drama, if the answers were that easy.

During this exclusive phone interview with Collider, actress Felicia Day talked about developing this character over four seasons, how the fan response to Charlie has blown her away, why she was initially interested in playing this role, the biggest challenges, that the episode has both emotional and funny moments, why Charlie wants to help Dean, and what she’d still like to see Charlie get to do. Be aware that there are some spoilers.

Collider:  What’s it been like for you to play Charlie and go through this evolution with her, across multiple seasons?

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Image via The CW

FELICIA DAY:  It’s been four seasons now. She continues, with every episode, to be more and more fun to play.

What’s it like to know that the fans love Charlie so much that they’ve petitioned to make her a series regular on the show? Is that something you would be game for, or do you prefer playing a role where you can pop in and out and make a big impact?

DAY:  The response from the fans to the character has blown me away and really made me so excited, every single time I get to come back to the show. It’s just a thrill, especially when every script is so strong and reveals an aspect of the character that adds to her, as a fully fleshed out person. Whenever Supernatural calls me and wants me to come back, I’m always thrilled to return to family, on screen and off. I can confirm that I will be returning another time this season, but I can’t say when or why. But I definitely love coming back to the show, in whatever capacity.

When you started out on this show, could you have ever imagined that the show would make it to Season 10 and that you’d still be making appearances on it?

DAY:  No, it’s crazy! I had always wanted to be on the show. When they called for that first episode, I was actually launching my company, Geek & Sundry, that month, and it was very, very difficult to carve out the time to do it. But there was really no question, once I’d read the script, that this character was someone I really wanted to live as. I thought she represented geek women in a way that really was authentic, and you normally don’t see that when you get offered roles. And the fact that it grew beyond one episode was so surprising, and I’m so grateful for the opportunity to be a part of the fandom and this show. It’s great.

What have you most you about playing this character, and what’s most surprised you, in regard to what you’ve learned about her, over the episodes you’ve gotten to do?

DAY:  I’ve learned a lot about Charlie, especially with her relationship with her mother and the depth to which her past is hidden. She’s never been able to really honestly connect with anybody in her adult life, about who she is and where she got where she is. And the biggest challenges have been those emotional scenes. There were a lot of challenges in playing the dark part of her, which impacted her in a real way that motivates her to come back in this episode. With the Book of the Damned, she can help Dean with the Mark of Cain sucking him into the world of evil.

This show essentially has two main characters, but the guest stars that come in and out have been so fully developed.

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Image via The CW

DAY:  Yeah, I know. There have been a lot of Charlie mythology episodes, that really focus on her with the guys helping her out. The fun part of this episode is her coming back with the Book of the Damned and evidence in tow, to really help solve Dean’s problem with the Mark of Cain, if possible. So, it’s not only a mythology episode, but she really feels like she’s working with the guys in a new way where she’s completely a part of the team. And she even gets to meet Cas. It was really fun to see her take that step forward into their world, when they’ve been in her world a bunch.

You’ve done funny episodes on the show, and you’ve done dark episodes. Do you have a preference for the type of storyline, or do you enjoy getting to do a balance of it all?

DAY:  Well, it’s definitely a challenge in both areas, the lighter and the dark. Robbie Thompson has written all of my episodes, so I always know that it’s going to be interesting, and it’s going to take me in a direction I never thought, and it’s going to be one hell of a ride. This episode is no different. I just love learning and peeling at the character, in these ways. It’s been fantastic.

Would you say that this episode falls more toward the darker side of things or to the lighter side, or is there a good balance of the two?

DAY:  I think it’s medium, in that it has a little bit of both. There are a couple of great scenes that are very emotional. There are some really funny lines, as usual. Charlie wears a great novelty t-shirt. What I do like for this is that it runs the gamut. I think everybody who watches the show will get something that they like out of it, which is really great.

What does Charlie think and hope that the Book of the Damned will be able to do, and how badly will that all backfire?

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Image via The CW

DAY:  We saw, in the last episode, that she went off to find this book that could have the tools to solve the Mark of Cain problem that Dean has. She’s highly motivated by having been split into the light and dark half of herself. We can all imagine what we’d do, if we were in certain scenarios, and how ruthless, violent and dark we could be, and Charlie had the rare opportunity to actually see that. She went through all of that, and I think she’s a little bit rattled by it. She wants to help Dean avoid the darkest part of himself. I don’t know if she likes herself quite as much as she did before, so it’s definitely a motivating factor for her.

If you could write yourself an episode of the show and have Charlie get to do anything and interact with anyone, what would you ideally like her to do?

DAY:  I would love to hang out with Castiel and the world of the angels with her. I think her perspective and the chemistry between the two would be super interesting. They definitely have different points of view of the world. That would be fun.

Supernatural airs on Wednesday nights on The CW.