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 '' The 15:17 to Paris '' , about the thwarted 2015 Thalys train attack . The film premiered on February 9 , 2018
2015 Thalys train attack
2015 Thalys train attack
Sinqua Walls