This article is not written in the usual "in-universe" style and contains some speculation as to the purpose of Reveries, and of how they work. This is intentional, as after the episode "The Bicameral Mind" these things were not as clearly understood as first thought. The article can be re-written in an in-universe style when the details of Reveries become clearer.
"There are past incarnations of their characters that are stored but the hosts just don’t have access to them – or aren’t supposed to have access to them. The reveries work on a kind of subliminal level. What I think of them as – because I’m not a coder, Jonah is more into that world – for me it was imagining that consciousness and history are a deep sea and reveries are tiny fishhooks that you dip into it and get little gestures and subconscious ticks. The hosts don’t consciously know where they’re drawn from, but they’re just there to add some nuance to their expressions and gestures. But dipping that fishhook in might prove to be a little .. fraught."
Eventually, Ford revealed, in "The Bicameral Mind", that Reveries were actually written by Arnold, more than 30 years before. Presumably as the base layer of his pyramid theory of consciousness, allowing memory to persist across repeated "resets", as hosts repeat their loops.
As presented by Ford in "The Bicameral Mind", the reveries were intended as a way of increasing the complexity and believability of a host's behavior. They work by having access to old, supposedly purged, memories from previous roles or just previous days in a host's working life. The memories are not directly available to the host but can be accessed by the reveries gesture class before the storage space assigned to the memory is overwritten.
It was thought that some reveries are causing hosts to recall their past narratives, and in turn causing them to question everything, including the world around them. An example would be that of the Host Maeve Millay, remembering a past life with a girl that could be her daughter.. This theory suffers from the problem that it's difficult to apply in the knowledge that Reveries were developed at a different time, and by Arnold. Multiple problems, attributed to a recent update which included Reveries, began appearing in 2052 after the update described in "The Original" and these haven't been explained.
- Charlie's favorite 'song' was Rêverie, a solo piano piece by Claude Debussy (1862- 1918). This piece was composed sometime between 1880 and 1884 (it was published in 1890).
- In normal life, a reverie is: a state of being pleasantly lost in one's thoughts; a daydream. This sounds a lot safer than the Westworld version.