How does Tolkien "let you fill in the gaps?" what are the gaps in TLOTR?
Lewis lets you fill in the gaps. Did you even read what he wrote?
i beg to differ. Tolkien might not let you fill the gaps in terms of description and the materialization of the world ... but his universe is so wide and populated that you are free to imagine all the relations between characters, between realms, between cities ....
you can fill in the gaps: economically, politically, love affairs ....
and The silmarillion is much more opened than the LOTR.