David Walmsley was born and raised in Liverpool, England. He originally wanted to study physiotherapy, but failed his biology O-Level exam and was encouraged to try drama school instead by a teacher who believed in his acting ability. Since then, that judgment has been fully vindicated. Walmsley's film credits include Ben Hur, The Terror, To Walk Invisible and Peterloo, although he is probably best known for his role in the Apple TV+ spy thriller series Slow Horses. Directed by Annabelle Comyn, Girl on an Altar was first co-produced in London with the Kiln Theatre last year. Also starring Eileen Walsh, it's the story of a royal couple who are torn apart by a terrible sacrificial act and an inability to forgive. "Agamemnon is a beast of a role, a beast of a man," Walmsley says. "[Having worked] with Marina Carr's writing before, I knew I'd be in for a treat."