flesh out

Also, put flesh on the bones of. Give substance to, provide with details, amplify. For example, The editor told her to flesh out the story, or You need to put flesh on the bones of these characters. This metaphoric expression, alluding to clothing a nude body or adding flesh to a skeleton, was in the mid-1600s put simply as to flesh, the adverb out being added about two centuries later.

The verb has 2 meanings:

Meaning #1: make fat or plump
  Synonyms: fatten, fat, fill out, plump, plump out, fatten out, fatten up

Meaning #2: add details, as to an account or idea; clarify the meaning of and discourse in a learned way, usually in writing
  Synonyms: elaborate, lucubrate, expatiate, exposit, enlarge, expand, expound, dilate