I try and make sure that all my minor characters have a potential to them, even if they are 'throwaway' (they aren't) in the part of the story that they appear in.
A previously major(ish) antagonist that I had planned to be dealt with by the end of book 1, ended up more sympathetic due to his own motivations (which I only explored fully as I wrote him), and became a small assistance towards (though still primarily an antagonist) the main characters and now has a small arc of his own that (unfortunately for him) is going to end up with him being dead in order to further someone else's goals and specifically set up events for future books.
Not bad for someone who started out as "generic gang antagonist" until I wrote him
Similarly, someone
else who was essentially drafted in as a replacement main antagonist as the other guy evolved into something more sympathetic, has gained a much deeper backstory, a full arc that leads through 'the bad guys', and will be a long term, though relatively minor, antagonist that will come to full fruition probably near the end of the planned 'book 2'
On the other hand, there are two characters I've purposefully seeded within the book who are intended to be very main characters later on (touching on one a bit more in book two, with both becoming a big deal in book 3), but for the moment are purposefully kept very minor.
Suffice to say, I try and make sure that even the incidental encounters have impact and relevance later on, but sometimes your characters surprise you and take over events as they develop