Building a profile for each character is a really important part of writing process. This helps you create characters that are ‘real’ to the readers mind. The more you know your character, the better you can write them. I find that when I’m character profiling, it also helps to have a photo of someone on the page. This can also help with what sort of personality you want to give them.
When creating a profile include their likes, dislikes, what they look like, their interests, past history etc. A lot of writers call this a ‘Character Resume.’
There are many Character Profiling templates around, but a good one can be found here http://www.writerswrite.com/journal/jun98/lazy2.htm. I've attached one of my completed Character Profile Sheet's here. It will give you a good idea on the amount of planning I put into developing my characters. If nothing else, the Character Profiling template will give you some good ideas of what to consider.
The thing I love the most about this exercise, is that I can make sure I don't create a bunch of characters that are too similar to each other. For a 'quick-add' Character Profile - I like to use this sort of template in OneNote: quick-add-character-profile.php
If you get really stuck on developing new characters, rope in the help of your friends. I've had to do this in the past. Send out the Character Interview Development Sheet and see what you get back. You will be surprised.