Spoilers for F is For Fake below.
So the big twist is that the last twenty minutes of the movie is fictional. It's also the most lucidly told portion of the film. I found myself not understanding or following a great portion of the first part of the film. The way the film is cut and put together is wild and rhythmic, perhaps mimicking the confusing nature trying to piece together a coherent story from real facts.
However, when the Welles starts to tell the fake story of Picasso, the editing style of the film shifts and slows down considerably. His train of thought style of telling the previous stories comes to a halt to focus on the Picasso story. This magic trick was incredibly effective because it conveyed the message and subject of the film without having overly explain.... until Welles explains it all. I only wish Welles did not reveal his magic trick to the audience so bluntly. But perhaps it was necessary to clearly demonstrate how the magic of a piece of art can be ruined by revealing all of the facts.
Facts can be boring and can contradict common sense and logic. Movies that are "based on a true story" are movies based on facts. Yet these movies rarely stick 100% with the facts given, because these facts can disrupt conventions of storytelling. Of course facts should always stay factual in real life, but when it comes to art, I feel that distorting these facts to fit an emotion or a feeling is necessary. Like sleight of hand, the true skill of an artist lies in entrapping the consumer in the art, by not giving them the chance to overthink what just happened because they are simply lost in the moment.