Well, documentary makers are cunning people and generations of them have developed refined methods for tackling this problem.
The single most obvious way to make the past come alive on screen is to use archive material. Luckily, film has been around for more than 100 years and legions of professionals and dabblers have used the medium to document everything imaginable, from the probable to the totally bizarre. In fact, the medium started out as purely “documentary”. Since film was so new and its visual impact so unprecedented, anything that was recorded was interesting to the viewer, even a train pulling into a station or workers leaving a factory. In the beginning, content hardly mattered: the novelty of the medium was the main attraction. Everyday situations, even the most banal, suddenly became interesting when brought alive on the screen. Back then, the director was a discreet, humble mechanic of the new medium. He wasn’t rushing around telling people what to do. He neither cut his films nor polluted them with his views. The director just recorded what he saw, purely for the sake of an uncanny effect. This was the medium’s age of innocence, and the visual documents brought forward to our day have the freshness and aura of truth.
Sadly, archive material did not remain uncontaminated and soon lost its innocence. People started realising the new medium’s potential. Once the novelty effect had worn off, content became increasingly important. As a result, filmmakers started to come up with all sorts of narratives and antics to keep their audiences happy. If a subject didn’t deliver, then the directors twisted it to make it work. The most notorious of these distorters of truth was Flaherty, the father of documentary filmmaking. In the opening of Nanook of the North, six people pile out of a canoe that is barely large enough to hold a single person. The film has hardly started, and the audience is already confronted with a situation that’s blatantly fake. It’s a small, humorous sequence, but it tells us: don’t take everything at face value. Flaherty was a true gentleman, and he played with open cards.
Unfortunately, later generations of documentary makers ceased to be gentlemen or-women. They twisted everything as best as they could but claimed they were delivering the gospel truth. They started to abuse the medium for shady political and propaganda purposes. Later archive material, particularly films from the Third Reich, cannot be presented as a reliable source. It is poisoned with the director’s views.