Whithouse offered a preview of Saturday's (September 16) episode to SFX, hinting that 'A Town Called Mercy' shows a different side of the Time Lord.
"To write him as a slightly more kind of volatile and unpredictable character was really refreshing and exciting," Whithouse explained.
He also revealed that one scene from the Western-themed instalment sees The Doctor brandish a gun, a marked change from his usual distaste for violence.
Whithouse said: "The sequence where the Doctor is kind of forced to use a gun, giving the right sort of emotional journey to that took a lot of finessing, and that was the scene that, from draft to draft, would change the most regularly because the Doctor is a confirmed pacifist and so putting him in that situation is a wonderful opportunity in that it forces you to confront it, and provides you with enormous tension and drama which, as a writer, is what you want."
The writer previously revealed that he did not use Doctor Who's William Hartnell-era story 'The Gunfighters' as inspiration for 'A Town Called Mercy.
"I thought about [watching] it, and I think either Steven [Moffat] or Mark Gatiss forewarned me and said not to bother," Whithouse revealed. "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.
Watch the trailer for 'A Town Called Mercy' below: