illustrate vs flesh out

illustrate and flesh out both are verbs.

Word	Noun	Adjective	Verb	Adverb
illustrate	No	No	Yes	No
flesh out	No	No	Yes	No

As verbs, flesh out is a hypernym of illustrate; that is, flesh out is a word with a broader meaning than illustrate:

illustrate: clarify by giving an example of
flesh out: add details, as to an account or idea; clarify the meaning of and discourse in a learned way, usually in writing

Other hypernyms of illustrate include dilate, elaborate, enlarge, expand, expatiate, exposit, expound, lucubrate.

illustrate (verb)	flesh out (verb)
clarify by giving an example of	become round, plump, or shapely
depict with an illustration	add details, as to an account or idea; clarify the meaning of and discourse in a learned way, usually in writing
supply with illustrations	make fat or plump