The box that Tiffin makes for Captain Oliver Woodward, as shown in the movie, was the actual box that was made in the trenches during the war. The Woodward family still has this box and gave the cast and crew permission to use it for the film.
A camouflet is an artificial cavern created by an explosion. If the explosion reaches the surface then it is called a crater.
At the church scene, all the soldiers are wearing service medals for the Great War (British War Medal and Victory Medal). Woodward and McBride are also wearing gallantry decorations of the Military Cross (purple and white riband). Sergeant Fraser is also wearing a Distinguished Conduct Medal (this is regarded as the level below the highest medal for gallantry, the Victoria Cross). One soldier is wearing a 1914-15 Star, indicating a longer period of service.
The tunnelers can be seen using modified cut-down Lee Enfield service rifles. This would have been done to allow the tunnelers to use a higher caliber weapon in addition to, or in lieu of, their revolvers.
The movie character of William McBride shares a name with the subject of a famous song by Eric Bogle entitled "Green Fields of France" or "Willie McBride", written in 1976 as Bogle visited a WWI gravesite and reflected on the war. The first line of the song is "oh how do you do, young Willie McBride". The song has been covered by multiple artists, notably Dropkick Murphys.