COLIN Morgan will cast his spell over fans during his first visit to Australia.

The 24-year-old has become a regular fixture on our screens as the young medieval wizard Merlin in the action adventure series of the same name, now in its fourth series on Channel 10.

With Morgan set to meet fans in Brisbane this weekend at the Supanova Pop Culture Expo, Merlin co-creator and executive producer Johnny Capps has given The Guide an insight into the latest series and why the unknown actor was chosen to play the famous wizard.

"(Co-creator) Julian Murphy and I always wanted to cast unknown actors for the four young parts," Capps said.

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"We were very keen to cast no-names except for Gaius (Richard Wilson) and we knew more or less we might get Tony Head (voice of the dragon).

"For us it was about finding the right actor for the part. We felt very passionate about that."

Morgan stars opposite fellow unknowns, or at least they were when the show debuted three years ago, Bradley James as Arthur, Angel Coulby as Gwen and Katie McGrath as Morgana.

But the young actors had to work for their breakthrough roles, said Capps.

"All of them, apart from Angel, must have been in 10 times at least," he said.

"We kept on bringing them back. When you're first meeting young actors they're pretty nervous. We looked for any glimmer of talent or spark and just kept bringing people back. Colin, Bradley and Katie all had something. We were all very intrigued by them."

The role of Merlin was the most difficult to cast and initially there were doubts about whether Morgan was right for the part.

"We had to find an actor who has a broad playing range," he said.

"Someone with the lightness of touch who will be able to deal with action and magic. There's a lot of green-screen work. We were looking for all of those qualities or the potential of those qualities, and the other important thing for Merlin was likeability. If the audience didn't like Merlin then we might as well go home. We had an instinct about Colin. At first we weren't sure about him but then saw him again and again and we watched back his final auditions and said, 'He's perfect for the part'."

The UK series follows the iconic characters of the King Arthur legend in their younger years, exploring the background to the often-told stories of their adult years.

Capps said the fourth series was all about building up to the golden age of Camelot.

"We see Morgana becoming more hell-bent on wanting her rightful place on the throne and Merlin desperately trying to prevent that," he said.

"And Arthur figuring out he needs support not just from Merlin but the knights. We're looking into the classic legends of the Knights of the Round Table and playing around a lot more with the legend of Emrys."

The producers have continued to improve the look of the show, shooting the fourth series with film-quality 35mm cameras.

"In series four the look of the show is more fantastical, more like Lord of the Rings in a way," he said.

"That was our benchmark in what we're trying to achieve."

The BBC has renewed Merlin for a fifth series, and that fits perfectly into the creators' overall plan for the show.

"Obviously, the BBC didn't commission five series straight away, but it was always in our mind to storyline and build five series," Capps said.

"After that it can continue, whether it would continue with the same cast or we do something interesting like go forward in time. We've got lots of ideas. The format of the show is robust enough to go on for some time, but at the same time we want to keep it fresh."

Merlin - Ten - Sundays at 6.30pm

Colin Morgan will appear at pop-culture expo Supanova at the Brisbane RNA Showgrounds from November 4 to 6.