Katie McGrath is better known for playing Merlin's evil Morgan, who is determined to overthrow her half-brother King Arthur and claim Camelot for herself. The Irish actress, who is from Ashford in County Wicklow, tells TV Choice more about the fifth series of the fantasy drama...

What’s Morgana’s agenda this time? Is she still determined to capture Camelot?
She does seem to have a knack for world domination, doesn’t she? But what she really wants is to be accepted. So if she rules Camelot she’ll be accepted for what she is and who she is.

Who are Morgana’s sidekicks this time?
Well, she’s sort of on her own. She’s obviously got Aithusa, the little baby dragon. But she’s pretty much alone, which is wonderful. She doesn’t need anyone. I like to think she’s pushed everyone away, and any relationship she has in season five is purely for her own gain.

But Mordred, the Druid who’s supposedly destined to kill King Arthur, is now back….
Yes, there was never any hope of keeping his return secret because his name was on the door of his caravan. But you can’t tell the story of Merlin without Mordred – he had to come back at some point. That’s the great thing about Merlin, everybody knows the legend – Arthur is going to be king and Gwen is queen – but you still don’t know what’s going to happen. So yes, Mordred is back, but is he the Mordred you expect? Probably not. So even when you think you know what’s going to happen, you really don’t. And Alex Vlahos, who’s playing him, is wonderful and I think you’ll be surprised and delighted with what they’ve done, what they’ve come up with.

Does this series feel very different to the last one?
It’s funny, but it doesn’t. Every year felt very much like a stand-alone series until four and five. But if you ask any of the cast now, they’ll say it feels like we haven’t been away because – even though we’ve got this time jump in the story – five feels very much like a continuation of series four.

Is it true you’re a big fan of Game Of Thrones?
I adore the books, but I made a decision not to watch the TV show, until I could watch a whole load of it together. I started watching it with my brother, but I got so upset that it was going to end. I’ll leave it until I can entirely indulge in 50 episodes. But I do have friends who are working on it, and part of me wants to say, ‘What’s happening?’

And Game Of Thrones star Liam Cunningham is now in Merlin…
Yeah, and I’ve known Liam for years, since I worked crew and costume on a job he did, before I was an actress. He wasn’t on Tudors, when I did that, it was Northanger Abbey and he did a day on it. And he knows my best friend very well. It’s Ireland – everyone knows everyone. It’s a bad cliché that turns out to be true most of the time. He’s a lovely man, and we’re so thrilled to get him. His voice is so melodious, it’s gorgeous.

How would you describe his character?
He’s kind of like a magic freedom fighter in it, and he’s like a revolutionary. Morgana and him have a tenuous alliance for a few episodes to get their own gains, so I have quite a lot of scenes with him, which are great.

As an actress, is it more enjoyable playing someone who’s evil?
So many people ask me this, and I feel like I should come up with a different answer every time. But I can’t – it’s bloody brilliant being bad. It’s so much more fun. I don’t think I’m a good enough actress to be convincing as someone good. That takes a huge amount of skill. Morgana was great when she was a princess and at the council meetings, but now that she’s slightly damaged there’s more scope for things in a scene.

What kind of scenes are challenging for you?
Anything in Old English. It’s so hard, and they keep giving me things that Colin Morgan did before – and he did brilliantly. And then I come on, and I’m like, seriously? But Colin gets three words in Old English and I get an entire incantation. I’ve no idea what I’m saying, and I’m learning it for ages to get it right.

Those scenes are hard, because they generally involve Old English and CGI. So you have to use all of your imagination. Because we shoot so quickly, you don’t know what the CGI is going to look like, so you’re imagining it. My imagination, though, is never as wonderful as they make it in the end.

BBC1, Saturday

David Collins