If you know how to write a likeable (but not perfect) hero, a dastardly yet sympathetic villain, and a reasonably complex plot, you probably think you're set. Unfortunately, this is not the case.
Unless your story involves only two characters (i.e. the hero and the villain) stranded somewhere desolate (such as a desert island, abandoned space station, or the Arctic Circle)...you are going to have secondary characters.
Secondary characters will share attributes with both your hero and your villain in varying degrees. For example, if your hero's attempts to arrive at work on time are thwarted constantly by the taxi driver who intentionally takes the route through heavy construction, then that secondary character has very few redeemable qualities.
However, on the scale of evil plans, postponing your hero's arrival at his job is far from earth-shattering. (After all, your hero could always hail a different cab further down the street, call the Better Business Bureau on the taxi driver, or take public transportation instead.) The villain, on the other hand, will be nearly impossible to avoid. Maybe the villain is your hero's boss, or a new client...someone that the hero needs to deal with.
When it comes to secondary characters, effort equals results. Sure, you can stop at describing the secondary characters by their job or their appearance, but then those characters will not have as meaningful an impact as possible. (I'm guilty of this in one of my novels - the main character's coworkers, friends, and neighbors are all practically non-existent. I mention that they do, in fact, exist...and then I get on with the plot that matters instead of ever meeting them.)
If, instead of glossing over your secondary characters, you let your hero have lengthy conversations with his waitress at the coffee shop, maybe she will warn him when she notices a car parked outside the shop, and thugs watching for when he arrives and departs. Knowing that information, your hero can dodge the thugs, fight them, or the waitress can show him a side exit. Then, the villain (boss or client) will not be able to use those thugs to track the hero's movements.
By simply giving your secondary characters the literary time of day, they can progress the plot and reveal your hero's personality at the same time. I've used a rather boring example here, but apply these principles to your own stories, and see if it helps!