A Narrator depends on three main things for life:
- The reality of the Story to set a personality
- The Story being told to call it into being
- The Story being spread to give it power
Each of these three, along with whatever limitations on the magic itself that restrict their creation, is a choke point for creating a Narrator force in the world. The stories literally develop a life of their own, but until called forth they cannot influence the world directly.
The core tenet is that every story can have its own spirit, and we invoke those spirits at our risk.
Premise 1: A Wild Narrator is a Minor Trickster Spirit
TL;DR: They can't have enough power to do much by their nature
The first idea is that a Wild Narrator does not have a lot of power because their stories do not have a lot of lasting power and are not grounded in a single precise moment of reality. They are the nebulous tales of minor events, sometimes exciting, that pepper lives that we remember and repeat endlessly through random chance.
Because these spirits are based on minor events that happen in a myriad of ways, they have been passed down through the ages like the larger tales. However they have not been passed down in a definitive line of succession, nor in any coherent way like a myth or legend would. This gives the Narrator lattitude in fulfilling its narrative and allowing it more freedom in its interpretation, but narrowing its scope to a single event.
Example: The story of a person getting injured when a small part of their ceiling fell on them has happened many times in history. However, it's a minor event -- a story that gets spread for a few days when it happens and then life goes on. Its spirit is not necessarily that strong as it is not about one single momentous event, but due to the many times and ways the story has spread, it can create the event in a myriad of ways.
Note: Not all tricksters are good -- I am almost sure enough people have fallen through ice and drowned that there is a Narrator Spirit for that too should somebody want to curse their enemies in winter.
Premise 2: Wild Narrators are Distilled Tropes and Genres
TL;DR: The greater Wild Narrators spririts are rarely invoked due to their nature
In contrast to the minor spirits of events, these Wild Narrators are the greater spirits that hold sway over tropes in general. They are the ones that are potentially invoked when a Storyteller spins a tale without it being a specific one.
In a sense, they have ascended beyond being the Spirit of the Story because they have been told and retold -- stories passed down through time until many mesh together into a sort of ur-tale. The Hero's Journey, being one of the archetypical ones, would fall under this category. Character Archetypes fall under here too -- religious mythologies tend to spawn them given the many changes and variations in myth and legend.
They hold an immense power within their sphere, but are incredibly hard to rouse because of it. It would require a story either so mind-numbingly generic that only the Overtrope could hold it effectively, or one would need to be able to invoke the ur-examples of the trope. Only the First Stories can reliable call the Wild Narrators of the Tropes and those are either incredibly secret or lost to the ages.
They are wild and capricious because these Spirits are so wide reaching and we can't control them by their nature. The tropes that they embody are much more vast than a single event and once set into motion, the Storyteller can influence the Story but not control it. However, a story usually resonaltes more with a spirit that is more specific to the story and these greate spirits rarely awaken
Premise 3: Getting Meta
TL;DR: Your Story is different, netting a different Narrator Spirit
By their nature, invoking a Narrator is exploiting the Fourth Wall -- this person is forcing a narrative onto something and they are holding the script that the spirit needs to follow. When invoked as a story proper, there is only a small level of risk. Namely that the spirit gets bored and/or creative when imprinting its tale on reality. But intentionally invoking a Narrator is also a Story and invoking this kind of metatale Does Not End Well.
For a start, the Story changes. It is no longer the tale of a town suffering from a drought that prayed for rain and got it. It is the tale of a person that wants to call the spirit of the rain maker stories and ask for them to being rains through the power of their Story. That is an entirely different Story, and they ways it can go wrong are immense. Not the least is that the Storyteller does not have the power here -- the Narrator does.
The biggest risk is a Narrator Spirit developing from those stories of people that invoke Narrators directly and knowingly. This spirit would have a similiar Fourth Wall Awareness as per the protagonist of the tale, only their awareness equate to the awareness of our world, and have the sentience to know that if people don't tell these stories they will fade into obscurity. Maybe it already has and the biggest lesson is Do Not Invoke -- Yes, the capital letters are mandatory.
Comparison: It would be like Deadpool being able to directly influence media people to produce more Deadpool stories so that Deadpool can always be on the shelf in some form. This world's version is probably saner than him, though no less aware of their own situation.