The characters of Tristan and Isolde, outside of "Merlin", are principal characters of a famous medieval love-romance, based on a Celtic legend. In short, on their journey home from Ireland, Tristan and Isolde, by misadventure, drink a love potion prepared by the queen for her daughter and King Mark. Once they drink the potion, the two are bound to each other by an imperishable love that dares all dangers and makes light of hardships but does not destroy their loyalty to the king.
The tragedy of Tristan and Iseult predated the Arthurian legends, and possibly inspired the Arthur/Guinevere/Lancelot triangle. And,like the Arthurian tales, the Tristan/Iseult tale has had so very many different versions over the years.
Ben Daniels (Tristan) has also worked with Joe Dempsie in the TV series Moving On (2009). Joe played the character of William (aka. Merlin's childhood friend) in The Moment of Truth (2008).
Ben Daniels (Tristan) has also worked with Karl Johnson in the TV series Lark Rise to Candleford (2008). Karl played the character of Taliesin in The Crystal Cave (2010).
Miranda Raison has worked with two actors who have played King Arthur. She worked with Bradley James in 'Merlin' and worked with Liam Garrigan in 24: Live Another Day, who would later play King Arthur in the TV series 'Once Upon a Time'.