OOC: Let's talk Backgrounds for a minute. The background is more than just flavor, but it isn't a full blown class level either.
Let's look at some examples.
Galreth and Vesper have the background of Acolyte, that means that at one time they were training to be Priests at a Temple of some god. They are knowledgeable of the followers of that god's rituals, rites and doctrines and will have Advantage in recalling and discussing those rituals and doctrines. Acolyte (p 194) NPC's are written to know Healing Touch (WIS Spell). DC 11. Heal one creature within close for 1d4 HP. In our recent set-to both Galreth and Vesper attempted to do healing, with, I assume, that Healing Touch ability and I allowed it, because it made sense to me that PC Acolytes should have much of the abilities of NPC Acolytes. If they want to succeed at any other Priestly Spell they will have a much, much higher bar to jump (higher DC, Disadvantage, need to make some major penance (even before attempting it, and/or spend a lot of time studying the ritual behind that Spell).
Now let's consider Gaurbal with the background Chirurgeon. You know anatomy, surgery, and first aid. Note that First Aid is listed and that is described as to stop a character from dying, or to Stabilize a dying creature so it doesn't need to make Death Saves. Now First Aid is a DC15 task, but a Chirurgeon should find it less difficult, so I might say "Roll DC15 at Advantage" if shaidar has Gaurbal attempts to give first aid. I'd say he gets ONE shot at it for the targeted character and failure just means he couldn't stabilize the target. However, what if he attempts that on a non-dying character? That's not in the book, but it makes sense to me that he could try it, and I'd probably give the same odds, but to only heal the target by 1 HP and, of course, failure NOW means he's can't attempt that again until after a long rest.
Or look at Morvith, with Cult Initiate as a Background. Cultists NPC's have Fearless and Deathtouch as abilities, it makes sense that Morvith should have some of that, say an Advantage to morale checks and the ability to attempt Deathtouch DC12, but only at Disadvantage.
Each Character has a Background and at worse they give them Advantage on rolls that relate to that background, and I can be convinced that a background applies to all sorts of things if the player works at it.

In addition, some Backgrounds have relations to other classes and the abilities granted those classes, I'm very likely to give each PC
low level access, maybe with some limitations, to those abilities. All of this is open to discussion between the Player and the Referee on what and how much leeway the player gets, of course.