fill-in vs peer

fill-in and peer both are nouns.

Word	Noun	Adjective	Verb	Adverb
fill-in	Yes	No	No	No
peer	Yes	No	Yes	No

As nouns, peer is a hypernym of fill-in; that is, peer is a word with a broader meaning than fill-in:

fill-in: someone who takes the place of another (as when things get dangerous or difficult)
peer: a person who is of equal standing with another in a group

Other hypernyms of fill-in include compeer, equal, match.

fill-in (noun)	peer (noun)
someone who takes the place of another (as when things get dangerous or difficult)	a person who is of equal standing with another in a group
	a nobleman (duke or marquis or earl or viscount or baron) who is a member of the British peerage

fill-in (verb)	peer (verb)
	look searchingly