From the 'Hero Complex' section of the LA Times:
‘Merlin’: Angel Coulby talks Gwen’s journey from servant to queen [excerpt below]. Slight spoilers for those who have not yet seen the final 5 episodes.
HC: Did you know the series was going to end the way it did?
AC: No, we never knew that. We were only told each series as it happened, and even then, we wouldn’t necessarily be told, because a lot of times the producers were very busy creating the show, so they didn’t always have time to sit down with us and tell us what was going to happen. And obviously also that would sometimes change. They must have had a vague arc in their head, but they never told us how the final episode was going to play out. That was just as exciting for us to read as it was, I’m sure, for the fans to see.