Some writers spend months or years delving into their characters. They write long biographies, interview the characters, set up psychological profiles, and figure out what type of ice cream they like. I have done that. Those are all wonderful tools.
But there are times you need to make up a character, sometimes a major character, on the spur of the moment. How the heck do you do that?
First thing is to think of what is the character's function in the story. Are they there to do an info dump? Are they there to add color, like a movie extra? Or are they there to help or hinder the main character? Will their role be large or small? When you determine how large the impact the character will have on the story, you can make up the character.
Minor characters, such as the shopkeeper, or the cab driver, can be your stock characters. Here it is okay to have the old guy selling the newspapers, or the teenage cashier. You don't need to develop any background for these characters. You just have to have them be more like a prop, not a philosophical being.
So what about the characters that take up a larger role?
I have a few tricks.
If the character is a helper, start with a caretaker stereotype, then give them a twist. Better yet, at least three unique traits.
You need a mother-hen like character. You decided on that because you need the protagonist to have help, but annoying help. So what would define a mother-hen character? Over-concern, nosy, they want to do everything for a character.
What type of twist would make this character unique? Making them male would be the obvious choice. But I want to do something different. So I make the character a counter worker at the local deli that the main character always eats at. She is a fixture of the deli, every regular knows her. But only a few become her “children”. The other regulars can get jealous of her attention to her “children” because she will give them free samples, or extra meat, etc.
There you go, a quick character sketch.
What about the antagonist?
Much easier. Just flip some characteristics of the character they are going up against. You have a strong male character, but he is nervous around women he doesn't know. He is smart and hard working.
Take one or more of his aspects and flip them. So you could end up with a strong male character, who is confident around women he doesn't know. He is smart and delegates work, so he doesn't have to do his work. You can flesh them out later, but at least you have a quick sketch to work with.
Another trick, is to figure out their archetype, and use the stereotypical example. But again, make a small twist, if they are going to have a larger role. Such as you need a wise, old wizard. But you make them a drunk.
The more you practice with coming up with quick characters the easier it becomes. Who knows maybe that one character you came up with, the one that helped your main character out at the toy store? Maybe something will click when you are writing. And you will want to develop them more.
Some tools to help you when you need to come up with a character quickly:
Collect stock characters. A good list is here http://en.wikipedia.org/wiki/List_of_stock_characters and here http://tvtropes.org/pmwiki/pmwiki.php/Main/StockCharacters
Read TV Tropes, just for ideas.
Have a list of random generators that you can access. I use Seventh Sanctum.http://www.seventhsanctum.com/
Naming a character: Behind the Name the best naming site.
Become familiar with archetypes: http://en.wikipedia.org/wiki/Archetype This will help you quickly assign the role of the character.