round out

verb

1. To become rounder or plumper.

   “It was Bill all right, but he'd really rounded out since I'd last seen him.”

2. To make more complete by adding details.

   “Here are some actual figures to round out the basic report.”

   “Mr. Burns is similarly perfectly cast as a heartless capitalist willing to do anything for a quick buck, even if it means endangering the lives of those around him and Marge elegantly rounds out the main cast as a good, pure-hearted and overly indulgent woman who sees the big, good heart (literally and metaphorically) of a monstrous man-brute.”