The Farscape actor will play cowboy Isaac in Wild West adventure 'A Town Called Mercy'.
"It's amazing that Doctor Who can attract these international stars," Whithouse told SFX. "He was fantastic. I was really, really pleased that we got him because it's an odd part - that character Isaac.
"Ben has this rather beautiful innate nobility that he manages to just convey, very simply and very economically. It was absolutely how I saw the character, so I was really thrilled with him, and delighted with his performance."
"I thought about [watching] it, and I think either Steven [Moffat] or Mark Gatiss forewarned me and said not to bother," he explained. "They said it's not exactly the jewel in the crown, so probably best to steer clear!"
Doctor Who continues this Saturday (September 15) at 7.35pm on BBC One. In the US, the series airs on BBC America.
> Doctor Who - 'Dinosaurs on a Spaceship' review
> Doctor Who: Have your say on 'Dinosaurs on a Spaceship'
Watch the trailer for 'A Town Called Mercy' below: