Characters are people, just like you and me. We all have our own little biographies and memories that shape our character, define our personality and predict and explain our behavior. Some of those biographies will inherently be more interesting than others and some will be engaging to only a few and not others.
The bank teller, the guy behind you in line at the supermarket or the young lady standing next to you in the elevator could be made of cardboard, as for as you could ever care, and in a story they could be. But they all have their own secret narrative.
I don't believe that you should set out to intentionally make your characters interesting or memorable. As a writer the vapors of your imagination coalesce into entirely new people that you then place into a situation of your choosing and like the silver spheres of a pinball bonus round they will careen and veer in an unpredictable behavior around the stage you've set for them.
You can add order to the chaos by providing every character of note (let's say all speaking roles) with at very least a mini-biography (main characters should have enough of a backstory to fill an hour on the Biography channel). Providing a back story will give you a way to predict the way someone will behave in a certain situation. Even if (especially, really) the reader will never know the backstory your plot cohesion will benefit immensely from it.
I guarantee that JK Rowling, JRR Tolkien, George RR Martin and anyone else who created an epic fantasy universe has notebooks and index cards full of important information about each and every character they ever created. You'll never see them but as a reader (and a writer) you'll appreciate that they're there. For instance, Dumbledore was gay. Rowling never said this outright in her books and left few hints about this aspect of his personality but I guarantee that somewhere in her house there is a notebook with the words "Dumbledore is gay" scrawled in pencil in the upper corner of the margin.
That being said, not all of your character's backstories will be inherently interesting, just as not everyone you meet has an interesting story. But they will be have one. And a character doesn't have to be lively to be memorable. Think HAL in 2001. That's a character who is completely devoid of personality. Yet he is memorable and interesting. Marvin the paranoid android would be remarkably boring to be around. Yet he is memorable and interesting.
Build your characters to a greater or lesser degree based on the level of interaction your readers and the main characters will have with them. Don't try to make them interesting. Just make them, and let the reader decide if they're interesting.
Since the question has morphed slightly since I posted my response allow me to provide a post script.
While I stand by the crux of my response (simply create new people and ensure that anyone worth mentioning has depth) there is a caveat or two. Readers will be drawn to some characters and not others, and that's okay. But if a character isn't interesting to you, he won't be interesting to your readers and characters that bore the reader give them an excuse to stop reading. But if there's a character who is intentionally boring then I believe he should be so mind-numbingly boring that he's simply unforgettable.