I had one of those real-life experiences that ends up teaching a writing lesson not too long ago. I was taking my recycling to the neighborhood drop-off center, and there was a sign at the center mentioning the recent passing of the center's attendant. I felt a real pang of sorrow, even though this woman was someone I saw maybe twice a month for just a few minutes at a time. Still, she really stuck in my mind, so that when I read the note about her death, I could picture her vividly.
It occurred to me that this is what you want to do with secondary characters in a book. They may not play a huge role in the story, and they may not be all that important in the grand scheme of things or in the lives of your main characters, but you still want readers to remember them to the point that they'd feel at least a little bad if something happened to them.
This recycling center attendant had some very strong, specific traits. The main one was that she took her job very seriously, even though it was the kind of job a lot of people would think didn't amount to much. The center has a bunch of bins labeled for different kinds of plastics, different kinds of metals, different kinds of glass, etc. Patrons are supposed to sort their own stuff and put it in the right bins, and the attendant was mostly there to keep things neat, keep people from just dumping their trash in the bins and to keep people from stealing stuff. But this woman didn't just sit there. She'd get up and make sure you were doing things properly. The first time I encountered her, I just had a few things, so I had them all in one bag and planned to sort them out myself at the center. I'd only barely started when the attendant came up and took my bag and started doing the sorting herself. At first, I was a little taken aback and worried that I was doing it wrong and was being judged and criticized (but then I think my CD player is judging and criticizing me when it keeps flashing "please insert disc" while I'm still juggling with the case), but then I realized that she was being helpful while making sure things were being done just so and probably alleviating boredom. I became a lot more conscientious about separating everything because I knew the attendant would insist on sorting it all for me.
In a book, your supporting character may just have one role, and in that one role, the character will be more vivid if you give that character one clear, strong trait that is demonstrated through action -- such as a recycling center attendant who is a stickler for sorting as opposed to one who sits in her trailer and listens to the radio. You don't have to develop these utility characters much beyond that. I didn't know anything else about this woman, like where she was from, where she lived, what her favorite color was, her favorite TV show, etc., but I did know how she did her job in the area where our lives intersected. It works best with just one strong, specific trait for a character in a limited role. An attendant who hung around making small talk about the weather and trying to get to know all the patrons while taking their bags from them and doing the sorting herself might have actually been less memorable than someone who was brusquely helpful. Pick one trait and really work it. The key for this kind of thing is action. Don't tell us the kind of person it is. Show it in the things the person does and the way those things are done, and then let us draw our own conclusions (or let the viewpoint character draw conclusions).
The test of your supporting characters would be to look at them and see if there's anything memorable about them. Could you recall one defining trait? The other side of this is that it the way your main characters react to these supporting characters tells you a lot about your main characters and helps define them better. Meanwhile, when the supporting character is crystal-clear, the scene will be more vivid and exciting.