You must juggle illumination & shadows, plus perspective and 'vanishing point'...

There's too many pretty 'scenes' over on DA where a character has been put onto rather than into a background pic, and just about everything looks wrong.
Still, gotta start some-where...

Perhaps you could take a trick from theatre, dissect that background into elements at different distances, apply those to 'flats' for 2 ½ D ?