Arthur's in-laws are played by Alexandre Astier's real parents, Arthur's brother-in-law by his half-brother, and Arthur's mother by his real-life stepmother. No nepotism here, though: they are all skilled actors, mostly from theater. He has also cast his children and wife in minor roles.
Merlin and Nathair (the spy) are played respectively by Jacques Chambon and Luc Chambon, who are brothers.
Christian Bujeau's son is an extra in the weaponmaster's classes.
The episode "Always" shows Arthur, Léodagan, Lancelot, and Calogrenant receiving a message stating that Perceval is dead, with the aftermath of that news. In the end it appears that he wasn't quite dead. That episode was written after an April 1st, when a journalist wrote an article announcing the death of Franck Pitiot (the actor who plays Perceval). Turns out, the journalist took for granted an Internet April's Fool.
The episode "Au bonheur des dames" shows for the first time Bohort's wife, Berlewen. Alexandre Astier created the character after reading in some newspapers that Bohort was "Kaamelott's crypto-gay", because he didn't want anyone to "write the show for him".
Also, there is a very rare example of subversion of this trope (which could also qualify as a Real LifeSmarter Than You Look): Kadoc, a character so stupid that he would be considered mentally deficient today, is played by Brice Fournier, a man who, before becoming actor, achieved a "classe préparatoire HEC" (a two-years preparatory class for entrance to HEC, a "Grande Ecole" specialized in business), a diploma in the Ecole Supérieure de Commerce of Lyon, a BA Management&Marketing in the University of Canterbury, a MBA in the Fordham Business School of New York, and a diploma in the Gemological Institute of America. Oh, and he opened his own restaurant in a wealthy and trendy neighbourhood of Paris, too.
Stunt Casting: A number of famous French comedians and actors have had cameos on the show, including Lorànt Deutsch, Élie Semoun, Alain Chabat, Guy Bedos, etc.