Determine the Purpose
Even in its simplest form, the structure of a frame story is more complex that a straightforward tale with a linear beginning, middle and end. This complexity adds needless confusion to the story if there isn’t a clear reason behind the frame narrative structure. However, in many stories, the frame narrative structure is vital to the telling of the story. When writing a frame narrative, you must first determine why the structure is necessary. For example, in “Frankenstein,” no one but the narrator Robert Walton is available to tell the story -- the main character, Dr. Frankenstein, has died, and the monster has vanished into the Arctic. On the other hand, in Joseph Conrad’s “Heart of Darkness,” Marlowe is telling his own story; however, the frame structure provides narrative distance, allowing Marlowe to reflect on his adventure.
Choosing the Narrator
Picking a point of view is only one aspect to consider when determining which character should act as narrator. You must also consider how the narrator's presence in the frame story affects the main story. For example, in frame stories in which the narrator is telling his own tale, such as “Heart of Darkness,” an element of suspense is missing because we know that the narrator survived his adventure; he’s lived to tell the tale. Frame story narrators also require special consideration in regard to their reliability as storytellers. For instance, it is clear that Shelley’s Dr. Frankenstein despises the monster he created, so the reader could consider Dr. Frankenstein an unreliable narrator and doubt his accuracy and truthfulness when he relays his monster’s story to Walton.
Given the complexity of telling stories within stories, it becomes necessary to first fully structure each story independently before weaving them together into one tale. If not, story details become lost or neglected, leaving the reader confused. Writing each narrative as its own independent story also reveals any plot holes and allows you to see how each story builds and arcs, as well as revealing redundant story beats. This exercise also helps to determine which character should tell each part of the story and when.
Since the frame structure provides narrative distance from the events of the main story, the narrator is in a position to comment on and give meaning to the tale being relayed secondhand. This commentary can be interjected throughout the story, allowing the narrator to clarify and interpret the tale as he tells it, rather than saving all reflection and moralizing for the end. Narrator interjections may also be used to foreshadow impending events and update the reader on events unfolding in the frame story. For example, in “Frankenstein,” the narrator, Walton, is sharing Dr. Frankenstein’s story through letters to his sister, in which he updates her on the unfolding of the frame story before continuing with telling of the main story.