Harry Melling stars as Gilli, who comes to Camelot to take part in the Kingdom's legendary tournament, in this week's episode of Merlin. The contest has one rule: there are no rules. Despite his small stature, Gilli has victory in his sights and a secret weapon to help him – magic.

As Gilli begins to realise the full potential of his powers, Merlin finds himself torn. Not only is his new friend putting his own life in danger, but his actions also threaten to rip apart the very fabric of Camelot itself. Can Merlin make Gilli see sense before it's too late?

Here, Harry reveals more about himself and his role to Press Information.

Do you watch Merlin?

I do, yes. I watched a lot of the first series. I remember when it launched and there was so much hype about it. What really stuck with me was the castle. I thought, "That must be computer-generated, I am sure a castle like that does not exist." And four years down the line there I am in Pierrefonds [France] filming at that very castle – which really exists in all its gloriousness. It's stunning, it looks as though it was dreamt up by Walt Disney.

What can you tell us about your character, Gilli?

He is quite innocent in the sense that he has never been to Camelot. This is his first trip there and I assume – well, I decided – he'd had a long journey to get there. I don't know what he did before but I think he was probably a servant – a bit of a peasant boy.

He comes to Camelot to try to prove something to himself and to everyone else. He decides to get involved in the tournament. When you first see this little guy he is in stark contrast to the huge warriors who have gathered to fight. You think, "is he crazy?"

As the episode progresses you see why he wants to prove himself. It's wrapped up with magic and his dad – it's all intertwined – and there is quite a messy reason for his actions.

Does he have magical ability?

He does, but not as much as I thought when I first read the script. Gilli has a magical ring – a bit Frodo-like – and through this ring he can channel magic.

The ring is actually Gilli's father's ring and his father never used it – which is key. His father wouldn't use the magical ring because of the dangers of it. That's the journey Gilli goes on in this story; he realises using magic is a truly wonderful thing – with regards to how Merlin uses magic. However, it is also a truly dangerous thing because it is so powerful.

Did you get to be involved in any stunts?

I did, yes. Because I was involved in a fighting tournament I had a little shield and a little sword and basically got hit by bigger men with bigger swords. It was great fun and beautifully choreographed. I did a lot of rolling about in mud.

I did the stunts in the first two weeks which was good because you always dread getting the big scenes first, because you never entirely gauge as to how your character got there. So it was great to do this very physical stuff first.

Do you have any favourite moments from filming?

The last day was special because it was just me and Colin [Morgan, who plays Merlin]. We had about three quite chunky scenes that happen throughout the story that really map out Gilli's journey.

What's fascinating about Gilli is that he starts off very innocent and pure, then when he starts using magic he becomes quite dark. Finally he comes out at the other end of it – and these three scenes really map that.

We'd done everything else – apart from these scenes – and it was all quite a rush because it was the very final day of filming. Everyone was de-rigging around us but it was lovely to have that time with Colin to work out the journey.

Merlin's brilliant, especially when you have the opportunity to do something that involves a lot of dialogue.

I guess Merlin and Gilli have quite a lot in common really...

They do; I think that was very interesting. Gilli and Merlin say to one another we're kin and they have that connection between them – and a definite understanding beyond anyone Merlin has so far come into contact with because there is a mutual understanding of magic. And Gilli's reasons for using magic make Merlin question himself, because he can see where Gilli is coming from. So it is quite an interesting internal battle for Merlin, questioning whether what he is doing is right or wrong. There had to be such an understanding between the two.

If you could have a magical power would what it be?

It would be clicking my fingers and being able to be anywhere "like that". I am a really bad traveller, I hate travelling and I hate being late so I figure if I could just click my fingers and be somewhere then that would be great!

You are reprising your role in the next instalment of Harry Potter; do you ever still get recognised as Dudley?

No I don't, which is really beyond amazing! I do look a bit different because Dudley was a very piggish character and about three years ago I lost quite a considerable amount of weight. It means that I can lead a normal life without the baggage of people running after me and shouting things at me. It was an active decision to try and change that and hopefully I can continue to do that throughout my career and be as mercurial and changeable as possible.

What was it like to be back filming with them again?

We actually filmed it a year and a half ago now, which seems like a long time ago. It was amazing to be back with the family and Dan because I don't see them regularly as everyone is off doing their own thing. When we do come together there is such a genuine gelling that it is so easy to get back into it. It is an odd relationship looking at it now, as we are suddenly together and suddenly dispersed but when you are together it just feels completely natural.

You said that you have lost a lot of weight. Will we see a difference in Dudley?

No, it was quite funny because I had a big fat suit on in this film and face prosthetics – interestingly by the same people who do Merlin's old face [from episode 10]. I am not sure how much Dudley will feature just because of the grand scale of the film and the fact that there are so many stories and characters to tie up. I haven't seen the film yet but I think it will be a blink-and-you'll-miss-it moment, but it was nice just to round it off and give it some closure.

Was the fat suit comfortable?

It was bizarre, you feel really big and my mobility was affected. However, it is amazing to feel light but within someone that should feel really heavy, so that was a fascinating contrast. It was interesting in terms of the physicality and thinking about how he would hold himself.

Is there anything else you have coming up that you can tell us about?

I am auditioning again – getting back to theatre would be amazing. I have had a really lovely year this year so it would be nice to repeat that. I've also just finished filming the role of Robert Brown in Just William, which is due to transmit on BBC One at Christmas.