When we think of movies that are completely true, we always think of documentaries. But there is no such thing as complete truth in films because its impossible to show it.
Documentaries are still the closest thing to the truth out of all the genres, but the moment something is given a certain perspective parts of the truth are blocked out, and when the footage is edited the truth is completely changed since it takes out all the parts the audience wouldn’t be interested in.
To show what I mean, I have a picture which shows how the truth gets obstructed the moment something is filmed.
This photo is from San Diego Video Production http://sandiegovideopros.wordpress.com/about/
As you can see the real truth is changed because of the people filming, but since they don’t film themselves filming, the truth is not shown.
Another thing that makes a documentary fake is the narration, since the person wasn’t actually there talking.
Many filmmakers have started trying to make films feel more realistic by acknowledging the camera, this is done using special effects such as lens flare and splatters on the screen. It makes the audience know it is a movie but implying it is real by making it feel more like a documentary.
So until someone makes a movie that has no editing, added sound, and shows everything it possibly can; documentaries are going to stay the closest thing to truth in film.