Andy Serkis said in the audio commentary he has no idea what's going on in the film towards the third act.
Many of the most horrific images in the film were inspired by a book called 'Covenant with Death'.
Torben Liebrecht had to be taken to the hospital to be treated for exposure after all the scenes where he's tied to a cross were shot.
Jamie Bell originally whistled "It's a Long Way to Tipperary," but the rights to this particular song proved to be too expansive so it was changed to "Silent Night, Holy Night" in post-production.
In the German dubbed version, the German soldiers speak French.