Casting director Pippa Hall was responsible for casting William as an extra when she was filling roles for Cider with Rosie (1998) (TV) back in 1998. Seven years later, when casting for The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005), she remembered him and recommended him for the role of Peter.

He had to repeat Year 12 because he had taken so much time off school while filming The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005).

From the first day of auditioning for the role of Peter Pevensie in The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005), it took a further eighteen months before he officially won the role and was cast.

He auditioned, unsuccessfully, for the role of Harry Potter.

As a 7-year-old, he used to listen to the Chronicles of Narnia audio tapes. He read the books eight years later in preparation for his role as Peter Pevensie in the film adaptation The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005).

He attended Sheepscombe Primary School between September 1991 to July 1998.

Grew three inches during the filming of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005).

Because of his major growth spurt, he had to have his armor modified weekly during the filming of The Chronicles of Narnia: The Lion, the Witch and the Wardrobe (2005).

He is the eldest son of Peter Moseley, a cinematographer, and Julie Fleming.

He has a younger sister Daisy (b. 1989) and a younger brother Ben (b. 1992).