Ability over Appearance: Given the time-period and the fact that Guinevere means "white" or "fair one", there were some raised eyebrows over mixed-race Angel Coulby getting the part of the future queen. The producers said that they had looked at hundreds of potential Guineveres, but Angel Coulby was the only one that could nail the quirky, clumsy servant girl, but also "bring the queen" when the occasion called for it.
Deleted Role: Emilia Fox as Morgause was all set to return after her death (at the start of season four) in order to appear to Morgana in a vision and give her the coin that she later uses to resurrect Lancelot. The scene was cut and never released on DVD, though there are some promotional shots out there.
Genius Bonus: Aconite, the poison Merlin uses to try and kill Arthur, also has medicinal applications so there's a good reason for Gaius to have some.
Also, Geoffrey of Mounmouth is the court historian. In real life, Geoffrey of Mounmouth was a record keeper- why does this mean anything? He's one of the early sources for the King Arthur Mythos.
In The Hunter's Heart, Gwen is transformed into a fallow deer. Fallow doe was a medieval term for young woman.
Hide Your Pregnancy: Emilia Fox was pregnant for most of season 3; it was hidden with her tunics but not always entirely successfully.
Romance on the Set: Bradley James and Georgia King (Princess Elena) started dating after The Changeling. Hilariously ironic given that their characters mutually agreed to call off their Arranged Marriage after coming clean with the fact that they definitely don't love each other. Apparently the actors felt a bit differently. There is also plenty of speculation surrounding Angel Coulby and Rupert Young (Sir Leon), considering their closeness on-set and their tendency to do a lot of publicity together.
Shrug of God: Good luck getting a clear answer on how exactly Merlin and Gaius are related. According to Richard Wilson, Merlin's mother Hunith is Gaius's step-sister, sister-in-law and/or his Childhood Friend. Elsewhere, the creators have said that he is Merlin's uncle - though haven't said whether this makes him a brother to Hunith or to Balinor. It's never been spelt out in the show itself.
Trolling Creator: The more fans complain about Arthur being conveniently knocked out, the more it happens on show. It's gone from only happening a couple of times per season to a Once an Episode occurrence, and has even extended to Merlin once.
Julian Murphy's DVD Commentary for the final episode. Good lord. Depending on who you talk to, he either confirmed Merlin/Arthur as canon or obnoxiously queerbaited for forty-five minutes. If you open up a discussion on the matter, be prepared for a Flame War.
Word of Saint Paul: Eoin Macken, who plays Gwaine, has mentioned that his character wouldn't react much if he discovered Merlin had magic. Given his gratitude towards the Euchdag and the fact that he only seems to truly hate sorcerers who commit crimes, this agrees with canon so far. Bradley James has also said that Arthur has reached the point where he's ready to learn about Merlin.
When it was announced that Bradley James and Angel Coulby would be be starring in a photo-shoot together for The Lady magazine (to supplement the Colin/Katie shoot of a few months ago), Colin/Bradley fans flooded the organiser's Twitter with rude demands about how they'd rather have Colin than Angel◊. The organiser's evident disgust◊ in her reply would indicate that they've effectively killed any chance they might have had in convincing her to do a Bradley/Colin shoot.
James Callis, who is best known for playing Gaius Baltar on Battlestar Galactica, not only played opposite a character called Gaius on Merlin, but rode a horse called Galactico.
Katie McGrath and Emilia Fox also played sisters in the TV movie The Queen. (Not to be muddled up with the film version of The Queen.)
Uther's forehead scar is made by applying makeup to a natural crease in Anthony Head's forehead-the crease is prominent in most of his Buffy photos.
Stories of the legendary King Arthur usually place the day in which he pulls the sword from the stone as December 25th. The episode in which this version of Arthur pulls the sword from the stone first aired on Christmas Eve, 2011. (Though if you're an international viewer, there's a good chance that you watched it on Christmas Day).
Make that four. They were in Manchild together, and actually do share a scene in which they briefly bond over a shared favorite movie — and this was three years before they were in Merlin!
Angel Coulby and Janet Montgomery (Guinevere and Mithian) never interacted during the episode in which they both appeared, however, they are going to share several scenes in the upcoming drama Dancing on the Edge.
Though Bradley James and Janet Montgomery also featured together in Dis Connected, along with Hunith, Sophia the Sidhe, and Sir Owaine.
If you're a fan of Miss Marple, then the ITV adaptations starring Geraldine McEwan/Julia McKenzie is a line-up of familiar Merlin faces, including Morgause ("The Moving Finger"), Nimueh ("By the Pricking of My Thumbs"), Valiant ("Nemesis"), Lady Vivian ("Why Didn't They Ask Evans?"), Lady Catrina ("Sleeping Murder"), the Gleeman ("Sleeping Murder"), Edwin Muirden ("Ordeal By Innocence"), and the Witchfinder ("By the Pricking of My Thumbs").
And from Poirot we have the Callieach ("Five Little Pigs"), Queen Annis ("The Mystery of the Blue Train"), Enmyria ("At Bertram's Hotel"), Edwin Muirden (again) and Princess Elena ("Halloween Party").
Emilia Fox (Morgause) is the Real Life niece of James Fox (King Rodor).