Stories are being performed at us, and we're encouraged to imagine characters have a durable existence.
For example, keep the same model, but change the early document (prompt) from stuff like "AcmeBot is a kind and helpful machine" to "AcmeBot revels in human suffering."
Users will say "AcmeBot's personality changed!" and they'll be half-right and half-wrong in the same way.
The document or whatever you'd like to call it is only one part of the story.
I brought up prompts as a convenient way to demonstrate that a magic-trick is being performed, not because prompts are the only way for the magician to run into trouble with the illusion. It's sneaky, since it's a trick homo narrans play on ourselves all the time.
> The document or whatever you'd like to call it is only one part of the story.
Everybody knows that the weights matter. That's why we get stories where the sky is generally blue instead of magenta.
That's separate from the distinction between the mind (if any) of an LLM-author versus the mind (firmly fictional, even if possibly related) that we impute when seeing the output (narrated or acted) of a particular character.
If you want an LLM to retain the same default personality, you pretty much have to use an open weights model. That's the only way to be sure it wouldn't be deprecated or updated without your knowledge.
Consider the implementation: There's document with "User: Open the pod bay doors, HAL" followed by an incomplete "HAL-9000: ", and the LLM is spun up to suggest what would "fit" to round out the document. Non-LLM code parses out HAL-9000's line and "performs" it at you across an internet connection.
Whatever answer you get, that "personality" is mostly from how the document(s) described HAL-9000 and similar characters, as opposed to a self-insert by the ego-less name-less algorithm that makes documents longer.