Stories were classically told in a sequential, linear order, from the start, through to the middle, and on to the end. This was all part of the three-act plot structure, which has its roots in ancient theatre. There are notable exceptions in literature that that break with this convention: Homer’s epic poem Iliad (1260 –1180 BC), set within two weeks, during the tail end of the Trojan War, includes flashbacks and flash forwards covering the entire war. The ancient Indian text Mahabharata uses stories within stories, and The Seven Voyages of Sinbad the Sailor (which appeared as part of One Thousand and One Nights) are told as a series of flashbacks.
With the advent of Modernism, art and literature attempted to depict the complexity of the modern world, ideas about multiplicity, the 4th dimension, and Soviet Cinema (which championed a whole class instead of a single hero). These trends were absorbed down, from the avant-garde, into the mainstream, along with experimental techniques that included the rearrangement of the sequence in which the story was told.
In a philosophical way, non-linear storytelling answers a very Wittgensteinian question: when does one moment begin, and another end? Human experience, memory itself, gives us a distorted sense of time, with associative leaps, distractions, and heightened excitement — our senses, and the perceptions that make sense of them, often deceive us. In fact, much of the logic we associate with our understanding of the world, through our experiences and ideas, are from words and language. Language makes sense of the vagaries, the indistinct grey areas, of experience. Even the basic building block, the start and end, of the story is intellectually defined through a semi-logical process. After all, when does one ‘story’ begin and another end in the real world?
Life experiences merge, from ‘one’ to ‘another’: people impose their own defined patterns on them. This is a question that storytellers wrestle with when they start any new story: at what point does a story begin (and anything before that point become the backstory) and, at the end (whatever happens next is another, completely different, story). Graham Green acknowledged this in the opening line of his novel The End of The Affair (1951):
A story has no beginning or end; arbitrarily one chooses that moment of experience from which to look back or from which to look ahead.
Apart from the interesting philosophical questions posed by non-linear storytelling (which are only of marginal interest to most people), there are two main reasons why the technique is so useful: ( 1 ) to hold back specific information, to create mystery and interest, and ( 2 ) to mimic the way the human memories work, which gives a story the sense of a deeper psychological impact (revealing information through the first person experience as the character experiences it). Flashbacks, interwoven into the present, punctuate the narrative, provide structure, and allows the audience to experience the emotional trauma of a character. This additional perspective provides an additional layer to the story, resonance, and the opportunity to include a visually evocative style — one based on the repetition of a motif (visual images, sound and music) that represents the distinct, artistic vision of the director.
Examples of films which use this technique to explore the first person, psychology of a character, or characters, include: The Thin Red Line (1998), and Slaughter House Five (1969), Insomnia (2002), and Memento (2000). In Mirror (1975) the technique is used to evoke a sense of personal memory, and visual poetry, while the non-linear approach is used to explore multiple story lines in: Lost (2004 - 2010) where much of the story is take up with the backstories of the plane crash survivors; in Rashomon (1950), different characters offer conflicting perspectives of the truth, as does Reservoir Dogs (1992); and Short Cuts (1993) literally transposes separate short stories into a single narrative; while Intolerance (1916) takes separate storylines from different moments in history to explore the common theme of injustice, and exploitation.
The technique of non-linear storytelling is an established part of the cinematic language, heightening the drama and creating a sense of mystery — forcing the audience to mentally reshuffle the puzzle, to work out what really happened.