I feel that I may be simply adding a footnote but here is what I do when the cast on the page grows really quickly.
I have a "bible" of background information which I tend to print off and carry about with me for when I have ideas. I add a list of characters to this. Now because I am adding a lot very quickly I cheat and use a short hand.
I try to add one of each of the following.
Character Flaw: Some vice, addiction or mental block that they can't see past. Sometimes this is just an excessive sensitivity based on a past trauma. My favourite character ever was once a walk on part that was utterly messed up from bullying when at school. He was so much fun to write that he got entire stories to himself and I do my best to throw him into the background of every story I write.
A Quirk: For really cheap characterization this can be a beard, a pipe they smoke, the over use of a word or phrase or just something equally mundane (like left handed or picks nose). It sounds cheesy but I have a list of quirks that I maintain and I try and pull one or two out for each character I create. It's not the daftness ofthe idea so much as how well you bring it to life that matters.
Morality indicators: Sometimes I simply use the DnD lawful to chaotic, evil to good scale but to be honest you should get a lot more exact than that. Supports slavery, is anti-gay, is gay, vegetarian, racist, is an anarchist at heart; etc. etc..
Loves and hates: I try to pick a bunch of random stuff (sometime literally random but sometimes better thought out).
A secret: This could be something like they are closet gay or they secretly vote right-wing but have lefty friends, they once got away with murder, they have a drug habit (see above lists). Maybe they are cheating on their wife or as someone else suggested they are a jock but they attend a night class in ballet. Are they in debt but too proud to tell anyone, for example (which leads nicely to the big question in a moment).
You can also add other stuff like if their mother and father are still alive and if they have siblings. Are they rich or poor? What job do they have? How do they dress?
There comes a point that for more minor characters there is far more information than you are ever going to show the reader and it is not going to make them any more unique to your mind.
If that still does not seem enough assign them a famous actor. This will give you an idea of voice and bearing that you are familiar with and some of that will come across in your writing. This also works by using people you know as the "actor".
Then we get to the big question that really brings all the fluff to life:
What Do They Want?
This is the thing that drives the character forward. In fact if the character is only going to hang about for a chapter or two this question alone makes them unique. What are they doing on your stage and why?
This is where those secrets, morality indicators and odd vices help you come up with consequences and motivators that fit. What crazy, foolish or funny situation is the character in and trying to get out of (or away from)? What are they trying to get? Also is it realistic or are they fooling themselves?
Are they just trying to get the shopping done before the wife gets home or are they trying to save their little sister from a drug habit?
TL;DR: Show me what each character wants by what they say and do and they will very rapidly become unique characters to me as a reader.