Adverse events result from life happening when the trial participant is taking a does or 2 of a drug a day, and is either spending the day going about normal business, or is in hospital (usually only cancer therapies).
The drug company consults with the FDA to decide when narratives need to be written. A general rule is if a trial participant is hospitalized or the event is considered serious, and the adverse event is known as a serious adverse event, or an SAE. The FDA wants to know if a serious adverse event occurs during a clinical trial, and the study co-ordinator is required to fax in or email notification.
You can see where this is submitted on the FDA website. This document is taken with the final data outputs from the case report forms and a narrative is compiled by the medical writer. These compiled narratives are included in the appendix of the clinical study report.
Every drug company has its own style of writing narratives. Here are 2 styles.