How to have objects begin behind a layer and move in-front of the layer mid animation

Hi I am a graphic designer, who is very new to motion graphics and animation. I am currently trying to learn basic character animation. I need to animate some people jumping out of a box, the problem is they need to start behind the box and later move in-front. As AE is a layer based program I don't know how to have my flat 2D characters begin behind the box and then during the animation move in-front. What is the terminology I have to google to find a video tutorial? Any help or tricks at all would be wonderful!