What makes a character memorable?
Think of your characters as people that are living and breathing, not puppets waiting for you to position them. Think about the traits you like or hate about someone and use them. Make your characters believable.
If you always wanted to go somewhere you have never been, base your story there. It will be worth the two weeks research.
What is behind your characters that motivate them to react, or do the things they do. Every single character in a book is motivated by something. If it's a husband or wife that has been cheated on, the motivation may be to kill the cheating spouse, and the person they were screwing.
What if they found out later, they had it all wrong? You now have the beginnings of a plot. It wouldn't be the plot, but it will work that way. In this scenario, I think the plot would be the cover up. To what extend would the character go to cover up what they did? Who would they involve, if anyone?
Give them an interesting back-story. Many people are cheated on, but don't choose to kill. Why did your character? I personally like to make it sick and twisted. I think it makes it interesting that way.
As a plus, I like to add an attractive man or woman in the mix, maybe both. He/she could be the detective or co-conspirator. Make them unattainable; it's better that way. (I think most of us can relate to that too).
Conflict: you can't let them get what they want. If they are trying to conceal their crime, find someone that will challenge them, a witness possibly.
Build suspense while you write and break the chapters just when something exciting is about to happen.
You know you'll keep thumbing though the pages to find out what's going to happen next. I'll keep writing until I find out too, lol.
Climax is a volcano that is ready to erupt and flood the village with molten lava . You should already have quickened the pace of the story to get here.
Resolution: should have everything brought full circle, and the reader should never be left wondering. Even if, you write a sequel, you should write a book, as it's a stand-alone. That way, you can never lose. I have read many books with the same characters, and I have jumped into a tenth book and it felt as if it was the first book they wrote of the series.
In conclusion if you ever get bored with what you're writing, kill one of you characters off.