Sure, you may have a great plot idea, and you may even be a good writer, but if you have boring characters you will find your story falling flatter than my pancakes on Sunday morning. Yes, they're that bad.
The thing about well developed characters is that even when you run out of ideas, they'll start writing the story for you. They will create situations you never thought of, bring their own interesting dialogue, and add that all important dimension that creates a personal connection with the reader. How do you develop strong characters? Start with a character sketch. Not a physical description, but more of a personality sketch (though physical details are included). This can be a list of items, a short description, an index card of notes, but something that describes in as much detail as possible your character. When doing this ask yourself the following questions:
What is my character's greatest fear?
What is their favorite food, memory, band, color, song, activity?
Where did they grow up? What extracurricular activities did they participate in?
Who was their best friend? Why was that person their best friend?
What is their little quirk, favorite phrase, or nervous habit?
The more questions like this you can answer, the more depth your character will have. Even if you don't use all of this information in the story, it all helps develop an internal sense of who the character is and what motivates them.
The final thing to consider is what is the one thing your character would never say or do. Have it? Good. Now put them in a situation where they have to say or do that very thing. That's how your character evolves. That's what raises the stakes and makes it interesting. That's what people want in a story.
I know this seems like a lot of work, but trust me its worth it. I used to skip this step then wonder why my story would hit a brick wall half way through. Now I start with a character sketch every time and I do it for as many of the characters as I can. It's brought my stories to life, and its fun to create your own people then manipulate their lives to your liking. Ahhhh, power trip. If only it worked in other areas too.
So go out there and build your little brainchildren, develop interesting characters, analyze and dissect them and then put them back together. You'll see your writing become better for it. And yes, you can also enjoy the omnipotent thrill it brings.