Build-up is the process of turning the box of seemingly-random parts into the actual finished and painted model.
It is never as simple as it sounds. The mold-making process can create difficult and uneven-looking seams (like the fingers of Frankenstein's right hand). And sometimes the accessories aren't realistic looking and need to made from from scratch (1:6 scale whip and sword blade for Indiana Jones).
The following build-ups show Brett's painstaking attention to detail.