Lancelot is known as King Arthur's right-hand man until it's revealed that he is having an affair with the Arthur's wife, Queen Guinevere, leading to the dissolution of the Round Table and King Arthur's betrayal. Osborne skips those later details in her books, and in this film Annie and Jack have to find this chivalrous knight when he is sent on a mission by King Arthur and never returns. Henry Cavill is the star of Zack Snyder's Superman films, but before that he had roles in historical dramas like The Tudors and Immortals. These characters would be good experience for playing a medieval knight.

Kiernan Shipka is best known as the bright and rebellious Sally Draper on the acclaimed AMC show Mad Men, but the world hasn't seen much of her since the show's finale last year. Kiernan is our choice for Annie, the series' other protagonist. Annie is brave and quick to take action—according to Osborne, Annie is the way the author wishes she had been as a child. The feisty personality we saw Kiernan display on Mad Men would suit Annie well.

Ty Simpkins appeared in Insidious and its sequel, as well as Jurassic World and Iron Man 3. We think Simpkins should play Jack, one of the main characters of the books alongside his sister Annie. Jack is less bold and confident than his sister, but more thoughtful and bookish. Ty's experience with blockbuster action films and the quiet intelligence of his character from Iron Man 3, the timid Harley who becomes an unlikely ally to Tony Stark, make him a good it for the role.

While debate rages on as to whether or not King Arthur was a real historical figure, he and his Knights of the Round Table may be the most well known characters from medieval literature. In the book on which this film is mostly based, Camelot is a joyless and desolate place and Arthur is discouraged after three of his Knights have gone missing. James McAvoy is currently playing Professor Xavier in the X-Men films, and with the long hair and beard he grew to play the young Professor he looks quite like this legendary king. 

These days, the name Merlin is synonymous with "wizard" and many versions of the character exist in film and television. For this children's book adaptation, we were inspired by Disney's The Sword and the Stone, where the powerful wizard is made more kid-friendly by imagining him as a bit silly and clumsy. Jim Broadbent has appeared in children's fantasy movies like The Lion, The Witch, and the Wardrobe and has played Santa Claus in two different films. We think he would make a great Merlin.

Morgan le Fay is a powerful sorceress in the Arthurian legend, depicted as an antagonist to King Arthur and a seductress who has multiple lovers, including Merlin. Considering that Morgan is one of the heroes of the Magic Tree House series and a mentor figure to Jack and Annie, Osborne's inspiration for the character most likely came from the 1983 novel The Mists of Avalon (Marion Zimmer Bradley's feminist retelling of the myth in which Morgan is the protagonist). Lena Heade, who plays Cersei, the powerful matriarch of the Lannister family in Game of Thrones, would bring these same strong maternal characteristics to the character of Morgan.