I'm sure there are multiple ways to do it; but what we do is just have the background layer in the scene, but not in the animation itself. Place a pause at the beginning of your animation controllers so that when the scene first is animated on, the background pops in, then there is a pause to await the animation of the remaining elements.
If a video background, the material will need to be set to auto start & free running in the material settings.
Hope that helps.

#XPression