The relationships began suggesting other relationships and characters.

All of a sudden I have a proliferation of characters: Secondary and tertiary characters popping up and saying "I exist, have a purpose and a function!".

It's inspiring but almost unnerving. Fortunately, I've just answered my own question about how to deal with this:

The premise.

Arrrr (piraty sound)

If they don't serve the premise then they can wait in the wings. (pun intended)

Thus any number of the characters will never be anything more than an influence felt through the way the visible characters act and interact.

I like supporting characters because they can ask questions, giving someone a chance to fill in gaps; they can give information; they can highlight character traits; and they can help reveal background or setting information. Among other things, I'm sure