Hundreds of people auditioned for the part of Merlin. They producers found imperative to come up with an actor with a broad range acting, who was able to do drama, comedy and to act in front of a green screen. They also wanted someone likeable, so the audience would tune in again after the first episode.
Before auditioning for the role of Merlin, Colin Morgan was sent the scenes for Arthur by mistake. He didn't get the whole script so he prepared those assuming they wanted to see him for the role of the prince. It was only right before for the audition that he got the scenes for Merlin so he only had like five 5 minutes to read over them. He ended up getting the role.