It's called darkroom skills, and the same concepts can also can transported into the digital world by the way. Depth in a print has mostly to do with contrast and every nuance, variation of it, between one or more subjects and backgrounds. There are those who simply stick a lens to 1.4 to create a depth of field play, and think that's what makes a great picture, and those who understand light, exposure, and again, more importantly contrast, to arrive at a beautiful print.