Merlin returns to Syfy for a fourth season Jan. 6, and no one is more excited than Katie McGrath.

While the 26-year-old Irish actress has loved playing Princess Morgana for the past three seasons of the international hit series, she tells Spinoff Online she originally signed on for the role because she wanted to play the character as the "bad-ass" queen of Camelot, wreaking havoc on the lives of Merlin, Arthur, Gwen and anyone else who stands in her way. And that's exactly where we are when the new season begins with the two-part episode "The Darkest Hour."

McGrath also teases that she has more screen time with Colin Morgan, who plays the title character, and sheds some light on the darkness growing within Morgana.

Spinoff Online: Are you walking with a bit of a strut these days because you are the queen of Camelot?

Katie McGrath: You know, there's that moment that you realize that you have a head that can wear a crown. And for every woman, that is an enlightening moment. You realize that you can no longer settle for mere mortals. You are a woman [laughs] with a crown. You are a woman with a tiara, and you need a man that is actually going to step up to that. So yes, absolutely it gives you a different swagger.

And I think it gives Morgana a different swagger, because now she has that confidence to rule. When you see her in Season 4, she is all about the confidence. She's all about self-reliance. She's all about driving herself forward. And she's all about retaining that crown because it looks so damn good on her.

Not all Merlin fans may know this, but after studying to work in fashion journalism, you landed a job out of university working on the set of The Tudors as a wardrobe assistant. And it was there you were discovered and started acting. I'm interested to hear your thoughts on Morgana's look in Season 4, and I was also wondering if you were actively involved in deciding some of her wardrobe decisions.

I wish I could take credit for Morgana's look in Season 4 because I absolutely adore it. She is sort of a goth Jessica Rabbit. She's not bad, she's just drawn that way. She looks wonderful. But we have a fantastic costume designer and, with the producers Johnny [Capps] and Julian [Murphy], they came up with the look that they wanted. I pretty much walked on set and walked into the costume, so thank God I loved it as much as I did because I had to spend eight months in it. I think they got it exactly right. She is just perfect as this dark, malevolent, evil but slightly sexual presence, which I think Morgana needs to be in Merlin.

Did you originally sign on for the role of Morgana because you loved how she was portrayed in Season 1, or was it because you knew what lay ahead for the character?

I signed on 100 percent with the idea that she was going to be the bad girl. Every year, I was like, "Let's get there, let's get there." So finally, Season 4 comes and I get to be bad-ass. I get to be controlling. I get to be confident. I get to be powerful. I get to be all the things a woman wants to be. And I get paid for it. I'm only sorry that it didn't happen in the first season. I'm only sorry it took three years getting there. And she was wonderful in all of those other seasons to play, but I've never had so much fun with her as I have this year.

Season 4 opens with a two-part, Lord of the Rings-style epic fantasy battle. Morgana has enjoyed lots of great interpersonal character moments the last three seasons, but is it fun to have the character let loose on this scale?

It's unbelievable. We started out as this little show in Cardiff with four people making this little thing that nobody knew about that was about magic. And for it now to be four seasons later and for BBC and everybody to put so much into it and show so much faith and build these huge-scale sets for us, you cast your mind back to the first day on set and see how far we've come you realize what an achievement it is. And just how hard everybody has worked. And how special it is to be a part of something like that. It's a humbling moment but at the same time, it's absolutely brilliant to be a queen with her own army.

Assuming she gets through this battle – and fictional history tells us she will – can you share any other teases for what's ahead for Morgana in Season 4?

I think Season 4 is going to be directly continued into Season 5. And I think in order to talk about them, you have to talk about them together. We're lucky. We've been commissioned for another season. It's safe to say that in Season 4 you finally get to see the beginnings of the legend as you know it. You get to see how Arthur may become king. And how Guinevere may become queen. Merlin is going to become this all-powerful sorcerer. And that's what's great about Season 4 for, as well. You start to see her as this strong, magical presence that everybody knows that she is. And that she ultimately must be. That's what you're going to get in Season 4.

You mentioned Merlin and hinted at the storm that's brewing between these two characters. Does Morgana get some extended screen time with Merlin in Season 4, and what's your relationship with Colin [Morgan] off-camera?

I don't like him at all. I keep rotten tomatoes in my dressing room and wait for him to walk by so I can pelt him in the face [laughs]. Actually, in Season 4, I have the most scenes with Colin than I think I ever have, which is wonderful because, to be honest, he makes me a better actor. He is so bloody good at what he does that you can't come in with an off-day with Colin. You know you have to hit the ground running. You know you have to be better than you are on your best day because no matter how good you are, he's going to be even better than you are. I'm lucky with Season 4 that there are quite a few scenes between Morgana and Merlin. And they are meaty scenes – we don't do fluff between the two of us in Season 4.

Is there goodness left in Morgana, or has she gone completely to the dark side?

I think that there is still goodness there. Morgana is still the same Morgana from the first season. She's not a completely different character. She's an evolution of that character. She still has all those bits of goodness, they're just buried so deep inside her because of what's she's gone through. She's running on pure revenge right now. She's so hurt and so damaged after discovering she has magic, discovering Uther's her father and ultimately what happens between her and Morgause in the fourth season but she is still – underneath it all, and I hope you can still see it because we've tried to keep it in there – a sympathetic character. She still is the Morgana from the first season. She's just on a different path.

You've played Henry VIII's mistress (The Tudors), Princess Margaret (The Queen), Lady Thelma Furness (Madonna's W.E.) and this holiday season, A Princess for Christmas. As an Irishwoman, what are your thoughts on playing these royals?

I'm such a tomboy. I'm the girl in a hoodie and a pair of boots, but I keep playing all these princesses. It's so ridiculous, if you know me. I'm nothing like it. And my family constantly makes fun of me for it. I'm the least-regal person going, but for some reason, I keep getting the parts, so I'm not going to complain. It's paying my mortgage and I adore it.