I've been watching "The Great War" on youtube. The footage they have seems to be a "reproduction" of wartime during WWI. What I mean by that is there is a lot of footage that has been taken from outside the trenches, looking down upon say, a machine gun shooting. Was this film actually taken during battles, or was it staged in some way?
...Yes.
There are plenty of examples of both staged footage and real footage of actual battle. The footage of actual battle tends to be filmed at long range because (obviously) neither side is going to allow the other to stand atop the parapet of a trench with the huge movie cameras of the time.
Consider the famous 1916 British documentary/propaganda film "The Battle of the Somme." Much of the footage is authentic--artillery and trench mortars firing, shells bursting over german trenches, troops moving up to the front, British wounded and German prisoners filtering towards the rear, etc. The footage of an immense mine exploding is also genuine.
However, there are plenty of staged shots as well. A particularly famous shot of men going "over the top" and falling back into the trench is believed to have been staged behind the lines--the "dead men" look at the camera and the trenches they're in are little more than ditches, hardly adequate for front-line protection. However, there is real footage of UK soldiers going over the top on July 1 and dying on camera.
You'll find a viewing guide here:
http://archive.iwm.org.uk/upload/package/100/Somme%20DVD/documents/viewing_guide.pdf
It's put out by the Imperial War Museum, who owns the copyright to The Battle of the Somme. The guide goes into a bit of detail about what scenes are perfectly authentic, what scenes are "real" but were filmed before or after the battle, and what scenes are faked outright. It's really interesting to see the film with the notes, so if you've got a spare hour....
You also have things like the 1919 French film J'Accuse, which did not feature footage of actual combat (it's not a documentary), but DID use real battlefields as sets, and used French poilu as extras in a climatic scene in which the dead soldiers rise from the earth and return to their home towns to chastise their loved ones for allowing the war to go on. According to the director, many of those soldiers were on leave from the Front, and were back at Verdun about a week after filming.
As far as I've ever read, German films were nearly always filmed behind the lines, but for that I have only the word of William Philpott's "Three Armies on the Somme" (more like "2.3 Armies on the Somme" if you ask me!), who I'm not particularly eager to cite. :/