I agree with Steph, sometimes added physical description can really enhance a scene and make it better. Other times,that description disrupts the flow of the story. It's true that anyone reading a BSB fan fiction, most generally, knows what the boys look like (except AJ, because he changes so often, lol, is anyone else expecting another hair dye stage soon?) Anyways, that type of detail is really useful in certain areas, of a story, to help the reader visualize the events taking place.
As for the background development...
I think that it's pretty important, most of the time. A short challenge story doesn't usually call for it, but a novel type story, almost, always does.
Sometimes, parts of that background are a secret, known only to the author, and may never be revealed to the readers. For example...in my story LATB, none of my readers know what is REALLY going on with Kevin. Kevin was a complete dick and no one knew why, for a long time, and when the story revealed a drug addiction it didn't go into detail about the cause of that addiction. My readers are still in the dark, left to assume the possible reasons. This approach can work in some stories and actually make it more interesting.
On the other side of the coin, a lot of times that background story has to be there, for the reader to understand what's happening. Steph is right about every author (and reader) veiwing the Boys, thier families, fans, ext differently. The presence of that background story develops each character, how you want them to be veiwed, and makes the story more believable. For example (from the same story) my readers wouldn't understand the concept of Nick being Baylee's guardian if the story didn't explain that Brian and Leighanne were killed in a car accident.
Okay, so basically, I figure that each story requires a different balance of these things. You just need to submerse yourself in the story and see what flows the best. I am still figuring out a lot of this stuff out, but I do find that each story requires different elements to make them work.
Did any of that make sense?
I sure hope so. If not, I know someone will tell me!