You've got Merlin coming up - tell us about your character.
"I think mythologically-wise he is Arthur's nephew. It doesn't work with me being Bradley [James]' nephew, so I'm now his uncle. You know what happened at the end of the last series? Morgana, Morgause, a big battle - obviously Camelot won. So we tidy up that and I become a presence at court. Quite a dark and brooding presence."
I like Katie's evil smiles. Do you get to do an evil smile every now and then?
"I do! I wouldn't call myself evil - in fact, we're not allowed to use the word evil - but I would call myself misunderstood. Only by Arthur. Actually, after 20 minutes you know exactly that [my character is] a baddie, but you're not allowed to say it too much, and obviously Arthur doesn't know. It's only Merlin and Gaius who really suspect, and really only Gaius at the beginning as well. That's great fun. Working with Richard [Wilson] is a treat, and Colin [Morgan]. Well, all of them, obviously. But Colin's a sweetheart."
Do you enjoy playing a villain?
"Love it, love it. I said when I started, 'Should I be Alan Rickman?' And they went, 'Ooh, nothing as big and broad as that'. Little did they know my baroque history of my baddie-playing! I do, I love it. It's good fun. I like playing good guys too. I like playing the bad guys to make them good."
Will you be in every episode?
"Yes, I believe so. I'll be there for the series, maybe more. I don't know if they're making another series - every year they say they might, they might not."
This series has been extended from the original ten episodes, hasn't it?
"13 episodes [now]! It's quite a long time."
But people love it.
"Quite rightly. I know a lot of people who love their Saturday evenings with that."
"He's avuncular. He tries to give him the best advice he possibly can. The truth is, he would run Camelot in a very different way if he had a chance. It's not that he dislikes Camelot, he'd just run it in a very different way. It's sort of the old guard - Arthur's so much the new guard - the old guard coming in. With some strength, I think. I think he's definitely a force to be reckoned with."
Did you enjoy filming at the castle in France?
"It's an amazing place. Great fun. Cantering up those cobbled steps into the gates, good fun. I love it. I love all that dressing up and playing around. We do quite a lot in Cardiff in a warehouse there and then in the Forest of Dean. It's a laugh, actually. It's quite extraordinary. I wish it was real! Somehow Merlin sometimes makes things look like a set - it's not! Actually we almost always use real things for exteriors."
Were you a fan of the show before?
"Yeah, I was. I watched all three series. My daughters love it, so I watched it with them."
How did they react when they heard you'd be in the show?
"So thrilled. When I got it I was absolutely over the moon because I knew they'd be able to watch it and they'd love it. This [Injustice] is harder for them to watch - I've got a 12-year-old and a 14-year-old so I'm not sure I'd recommend the 12-year-old to see this. Or the 14-year-old! But that, yeah, love it."
You've been in Stardust and Narnia - is the fantasy genre something you're interested in?
"Yes. Clive Barker was an old chum, who wrote Books of Blood and the Hellraiser movies and things like that. He was a colleague of my brother's - they worked together on fringe theatre for a while. I did used to like fantasy. It's not something I've grown out of - I enjoy it still. I'm not a fan of any one particular genre, but I think getting people out of their shells and doing something magic is always a top thing to do."
The fourth series of Merlin is currently filming in the UK and France. Nathaniel can next be seen in Injustice, starting this Monday at 9pm on ITV1.
Thanks to Digital Spy!