Fixing the Flesh Wounds: Healing & Healers, Part I

It's a dangerous world out there in Fictionland, and characters do have a tendency to get hurt. Interestingly, the first-aid skills of the characters around them and the perception of what is a survivable wound in this setting appears to vary wildly in accordance with the plot. Disposable side characters die instantly from relatively minor illnesses and injuries (or a bit longer and tragically, as the case may be), while lead characters can survive massive trauma with barely a scar (and all of their brain cells and internal organs intact).

This can be reasonable if there's a big difference in environments or circumstances-- for example, one character is much closer to medical help, is in a harsher physical environment, has buddies who are good at first aid, or is in a time and place where medical science is largely undeveloped. However, when characters in the same environment have similar diseases or injuries and one shakes it off while the other suffers instant death, it reeks of authorial meddling.

The way to avoid this is to do some comprehensive study of what medical care is like in your setting. What is the level of medical knowledge and ability? Does everyone have access, or is access limited by things like socioeconomic class or location?  Which characters, if any, have advanced medical skills? Which characters, if any, have any medical conditions which will effect their care? What in the environment might influence outcomes (temperature, disease-carrying bugs, etc)? Once you've figured this out, you can plausibly have different outcomes for different characters with a solid in-universe explanation.

Finally, it's also worth noting that the rules of the fictional universe should apply to your main characters as well as the minor ones. That means no scrubbing away all consequences of an injury or illness. The character should scar just like everyone else, and not get any special healing perks. Realism will add suspense, as your characters are in genuine danger of harm.