Playing the dual part of harassed schoolteacher Donald Peterson and his 'golden boy' brother Roderick, Tennant fills a similar role to that of Martin Freeman in 2009's original Nativity!.
"I watched [the original] and thought, 'There is something really special about this', something really direct and big-hearted," Tennant told Digital Spy.
"It has a kind of wonderful naivety to it, which you don't see enough of, and I thought that was something worth being involved in."
"How often do you get to do something quite so camp?" Tennant joked. "But because it's improvised, there's no dialogue written down, and that can be difficult in a scene where the brothers meet.
"It was great in terms of preparation... you just turn up and hope for the best. But it was a white-knuckle ride very day, because you don't know where it's going to go."
> 'Nativity!' review
Nativity 2: Danger in the Manger is released today (November 23) in the UK.
Photo gallery - Nativity 2 premiere in pictures: