To each character, a purpose

As I was saying before I was so rudely interrupted by a discarded character tirade...

Gamnock will now illustrate-

"How can I illustrate anything? You have me bound the desk with masking tape. I can barely move!" He struggles against the sticky layers.

"I meant figuratively, dear Gamnock. Now hush, or I'll get the tape out again."

I'd sort of learned my overpopulation lesson by the time I'd reached the near conclusion of book one. I thought I'd just toss a few characters in, like Xander and Gamnock, establishing them in book one so they would be ready to go in book two.

Great idea, but what if someone didn't read book one? I mean, these should be stand alone novels, right? Tossing in new characters toward the end that don't play into the main plotline makes for loose ends. They're just hanging out, screaming to the poor reader, "Tune in next time to see why I exist!" The reader doesn't want to pause to think about next time, they want to sit back (or hopefully perch on the edge of their seat) and enjoy the conclusion of book one.

Besides, I have enough work ahead of me in re-establishing the central characters that are vital to the story. I really don't need any more work and words wasted supporting characters than absolutely necessary. We want to leap into the sequel's plot for goodness sake, not have four chapters of refresher material.

Gamnock slams the backside of his boots on the desktop, making a horrible racket. "So I'm not memorable? Is that what you're saying?"

I whip out my handy roll of tape and secure his feet. "I said, shut up."

Ahem.

To prevent more Gamnocks - who really is a good character, by the way -

"Why, thank you."

I rip off a piece of tape and dangle it over his face.

"Right. Sorry. Shutting up now."

I now consult my handy checklist whenever the urge to toss someone new into the story strikes.

-Are we in the later half of the story?
----Do we really need someone new now when we should be focusing on who and what we've built up in the first half?

-Is this a bit part?
----Can it be filled by someone already established instead?
----Does this bit part player really need to become a full character?

-Is this character set up for something that doesn't even happen in this particular story?

-Is this character being planted in the opening chapters and then will not be mentioned again until a big reveal near the end, where no one will remember them because they haven't been mentioned in twenty-six intense chapters packed other drama and action, and thereby will totally spoil the reveal for everyone but me?

-Is this person going to be killed by my MC within the next few paragraphs?
----Haven't they killed enough people to make the point that they kill people already?
----Does the killing have major impact on the MC and plot?
----Would it have more impact if s/he killed someone already established?

-Isn't there someone just like this in the story already?

-Are we in the first quarter of the story? Will this character play into the plot later, or am I wandering off on an unnecessary side plot?

-Is this person appearing out of nowhere and now is going to require me to go back and establish them earlier on?
-----Will it be worth the work and/or screw up other elements of the plot?

"Good thing, you didn't have that list when you started," Gamnock says quietly.

I consider reaching for my tape, but he's got a point. "We have learned a lot together, haven't we? Besides, it would be awfully quiet around here without all of you."

"So you'll let me go now?"

I slip my fingernail under the edge of a strip of tape...

How about you? How do you pick and choose when and if to include new/unplanned characters?