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.
Producers liked Katie McGrath's audition but were hesitant to cast her due to the Irish accent. But since Colin Morgan was able to do an English accent, this allowed them to cast Katie and have her use her natural Irish accent.
Also Colin Morgan has portrayed a total of three different characters including standard Merlin, Dragoon/Emrys, and Dolma. How he manages to pull off making all three different characters with different quirks is amazing.
Aluminum Christmas Trees: The legends of Merlin are younger than the legends of King Arthur. If they two lived at the same time, it would have been when Arthur was an extraordinarily old man and Merlin was just a boy. So the age difference of the characters is, if anything, not wide enough.
Billing Displacement: Angel Coulby is credited before Bradley James in the opening credits; a little strange considering Arthur is the show's Deuteragonist. Speculation is that Angel went first because she was the most established cast member at the inception of the show, or that the names are simply in alphabetic order after Colin Morgan.
In the episode Another's Sorrow James Fox is ranked before Janet Montgomery, even though Montgomery is in nearly every scene of the episode and Fox's role barely constitutes a cameo.
Channel Hop: In the USA, the first season aired on NBC during the Summer of 2009. From the second series onwards it was transferred to fellow NBCUniversal-owned network Syfy, where it faired much better than it did on broadcast TV. It stayed there for the rest of its run.
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.
Dyeing for Your Art: Katie McGrath is naturally blonde. She dyed her hair brown and "stays out of the sun" to play Morgana.
Fake Brit: Colin Morgan is Northern Irish and affects an English accent for Merlin. Katie McGrath as well to a smaller extent, as she keeps her own Irish accent - and neither of her parents ever appear on the show, so one could assume they were Irish too.
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.
Out of Order: 3X08 ("The Eye of the Phoenix") and 3X10 ("Queen of Hearts") were switched around in order to boost ratings, which did some pretty serious continuity damage to Morgana and Guinevere's relationship. And ironically, "Queen of Hearts" ended up being the highest rated episode thus far.
2x07 ("The Witchfinder") was originally meant to be 2x04, but was pushed back.
Emilia Fox usually played doe-eyed Woobies in period dramas, or else a forensic doctor on the show Silent Witness. A Lady of Black Magic like Morgause was quite different for her.
Charlene McKenna, who plays Lamia, was best known for her role as a Hard-Drinking Party Girl on Raw or else other Genki Girl roles. Seeing her as such a dark villain is extremely different.
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.
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.
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.
In interviews Katie McGrath was pretty adamant that Aithusa was female, though the only mention of the White Dragon's gender was when it was first introduced, when Kilgarrah refers to it as "he."
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.
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).