Who are these people, were did they come from and how well do I have to know them?
Pretty well, I'd say. Your book is about what happens to people so you'd better know who they are why they are in your book and why all these things are happening to them.
First of all, you have to find names for them. That may be harder than you think because it depends on where the story takes place. Have your characters always lived in Minnesota? You're going to have Jennie Larson and Brad Nordhal. California or Florida? Jesus Rodriguez and Maria Cruz. Chicago has a large Polish population - so, do a little research and name your people accordingly. But don't limit yourself either. Folks move around and marry so vary your names.
There are a lot of baby naming books that I find fun to read. The one I have, "The Character Naming Sourcebook" from Writer's Digest Books, lists names by ethnicity and that's a help. I have gotten names off the covers of books in my office, off boxes of food and my favorite, the obituaries. If I read that John Smith and Bill Jones have left us, I don't use their names exactly. No, I might use the name Smith Billings.
Good guys and bad girls - please make them believable. No one will be interested in a villain who is totally bad, that becomes boring. Give him one good trait - that will make him more interesting. Maybe he loves cats? He's been on a rampage and killed two men, but he has to hurry home to feed and cuddle his kitten. Same thing with your good character, give her a bad habit. Perhaps she chews on her fingernails and cuticles. She's gorgeous, slim and well-dressed, the kind of woman we love to hate. But wait! She's got terminal dandruff! She's human! Give your characters a tag - that can be fun. Not just flaming red hair. How about a wart on the side of his nose that he fiddles with when he's stressed? Did she inherit a long lower lip from her grandmother? One of my characters, (Stormy Love) has two different colored eyes! She's been self-conscious of it all her life, thinks she's ugly and never realized how beautiful her eyes are.
I don't know about you, but I'm SOOO tired of the burnt-out alcoholic cop. It was an interesting concept when it was first used but now it seems like every other book has the same character - or the female version. I know it's hard to come up with something unique, but so is writing a good book. So search your mind to find someone different. I remember a character, a private eye who was a dwarf or little person. His normal sized brother was a cop, so he could funnel information to the PI to help. He was interesting and different. Try for a character we've never seen before or one that has traits and habits we don't read about in every other crime novel.
Many books and several sites on the Internet have character charts you can fill out to get every little detail of Sally's life. Beside the usual skin/hair/eye color, height, age, etc. you can go into her background. Who are her parents? Where was she raised? What church does she attend?
Who are her friends and why? What are her fears? The list goes on and on and you can spend days, weeks, months getting to know your character - but do you have to? Only you can say for sure.
When I get ready to people my books, I get the basics down. It always amazes me how much I "learn" about a character once I start to put him or her on the page. They usually tell me what I want to know.
I always love it when a character simply shows up on the page and won't leave. I've found that to be the case in every book I've written. I'm not sure if my subconscious is at work or if there is a writing gremlin somewhere that's helping this poor writer out, but I do know this: everyone one of those characters enriched the books they chose to inhabit.
One of my big pickies is weird people names. Of course you don't want all Bills and Sallys and Janes and Marks. But please refrain from using something so weird, no one can pronounce it. Which would you rather read about - Anentor or Anthony? Panphilia or Pamela? Remember, every time your reader comes to something he doesn't understand, it's like someone is standing behind him ringing a bell. Once that bell is rung down goes the book. That's the reason I've never read a P.J. James book - I still have no clue how that man's name is pronounced and it stops me every time! Dalgliesh - ding, ding, ding - I still don't have a clue.
Of course, like rules this one can be broken. In my book, "The Colors of Death" I have a Calligenia and a Phaedra BUT there's a good reason for those names, and they're only used once.
You have your characters all named and ready to go; you know who/what/where/when and how and certainly the ending. You've planted red herrings but you're still not sure you have enough for your mystery. Or, maybe you only have an idea, the smallest germ of an idea for a mystery novel but don't know how to flesh it out. You might want to try mind mapping. Basically put a key word or sentence in the middle of a large sheet of paper and draw a circle around it. It might be, "The priest dies at midnight." Now, draw a line straight out and put down the first thing that comes to your mind - poison. If something pops into your mind about poison, draw a line out from there and write it down, cyanide. If not, go back to the middle circle and draw a line in a different direct and write the first thing that comes to your mind - defrocked. Go on this way until your big sheet of paper is filled with lines and circles. You'll be amazed what's lurking in that head of yours.
If you want to see the technique in action, I'd suggest you go to the Internet and look up, "The Snowflake Method," by Randy Ingemanson. He's got a marvelous website that will show you in detail how to use this method to outline and eventually write your novel.
Or if you have most of the mystery plot outlined but are only stuck on one point, you can do some free writing. Get a piece of paper and pencil or pen, set the clock for five minutes or ten or whatever. Think about why you're stuck. You might even want to put a word or two on the paper. Okay start the timer and write. Do not stop, do no punctuate, do not THINK - just write. When the timer goes off, read what you have. It may be garbage, but it might be brilliant. You can use this method over and over and you will be amazed at what's lurking in your mind.
Still stuck? Don't give up - give yourself a pre-sleep suggestion. Just before you fall asleep at night tell yourself, "I have to find out how to be Betsy out of the locked room in the basement." Go to sleep and in the morning don't be surprised if the answer comes to you. It might come in the middle of the night so I'd suggest you keep paper and pen - and maybe a flashlight by your bed. You may think you'll remember in the morning but trust me, you won't. Matter of fact, I keep pads of paper and pencils all over the house so I can doodle down thoughts anytime they come to me.
