Readers love characters - they are our connection with the plot, what makes us reopen the pages, what makes us care.
I sometimes hear writers say, "Well, she's a middle-aged woman with an empty nest and feeling lonely." Okay, that tells us her current circumstance but it does not tell us who she is. And who she is depends upon her past and how she now looks at her past. Has she changed? How has she changed? Have those around her changed? (Think Shirley Valentine). And before you decide to insert a block of background on your main character, please note - no information dump, ever! There are few things worse than the background information dumped into the early chapters of a novel forcing us to wade through the character's early years.
The Poisonwood Bible by Barbara Kingsolver has a family of characters who change so subtly that, as we read we see the transformation before us - and we care, deeply! Similarly, The Mosquito Coast by Paul Theroux has a father character who, at the beginning, appears likeable but midway transforms into someone threatening. It's this seamless quality of transformation that is interesting and it's worth examining the early sections of these novels to find out what subtle clues were planted.
Some characters change little through a novel but that is their role. Bridget Jones is hugely popular and someone we feel we know. Why? Because she bared her insecurities. Few things can tell more about a person than what they fear. 
When coming up with characters think about acquaintances, those we barely know. Now imagine asking them for their Desert Island Discs - those songs that mean something to them, that tell a story of a segment of their life.
Take your character outside of your novel, how would they cope with a disaster on the street before them? How about a sudden illness? A lottery win?
Your novel may not be about these things but know how your characters would respond.
8 questions for your characters:
1. Tell three songs that make you reflect back on your life?
2. Regrets, have you had a few?
3. Any irrational fears?
4. Who would you donate a kidney to if they needed it?
5. How would spend a lottery win?
6. Do you polish your shoes?
7. What makes you happy?
8. Who is your favourite character in a novel?