He starred in the 2011 horror feature "Shark Night 3D". Walls played a guest role on the second season of the ABC fantasy series "Once Upon a Time", as Sir Lancelot. He returned to the show during the fifth season as a guest star. He played the role of Shawn in the TV series "Power". On August 1, 2017, Walls was cast in Clint Eastwood's biopic "", about the thwarted 2015 Thalys train attack. The film premiered on February 9, 2018.