I read this a lot on writing boards and writer chat areas. Authors go with something like, "If I have a real good story idea (which I do), then how important are things like characters?" or "I'm not the greatest at writing characters - how do I get around this?" The writer in me dies a little when I see questions like this, and I try to answer them as politely as possible. However, it boils down to a very simple position - characters are the story. They deliver it, they play it out, they carry the weight. There's no way to "get around" the character situation.
Just like how an artist will often do several sketches of their subject before they paint the portrait, writers should write out a few narratives of their characters to better understand them; to get a feel for how they feel and respond. For every manuscript file I have, I have several smaller write-ups of the main characters. Simple descriptive paragraphs about how they walk and talk, their quirks, their phobias. Little character sketches about them enjoying a hobby or preparing a meal. Even small short stories putting the character through some odd encounter to see how they respond. If I write these, I know the character better for doing so. If I have trouble with these, maybe I need to know my character better.
A trick that I use these days was actually inspired by social media. You know those occasional questionnaires people circulate asking about your favorite food, favorite color, social security number, etc.? Well, instead of filling it out on your own and getting your identity stolen, fill one out for your character. Try to understand the little things about them - the things that mostly float around inside their head but are rarely discussed. Knowing these things starts teaching you about personal motivations and drives, and allows you to build around the things you already know. A character with a lot of past trauma might have it come out in odd ways, and once you know those ways, you can give your character that much more depth.
Lastly, I try to write a sketch of that character encountering something that challenges their beliefs. If they don't believe in ghosts, then write about them seeing something ghostly and how they process this information. In life, we learn a lot about people in moments of crisis - writing is the same way. We give that character a moment of internal conflict and see what happens. In doing this, the character grows, and we understand them better.
Hopefully, this offers a little help in the character-building process. My secondary intention is to reply to people's character inquiries with a link to this post, but this is mostly another way for us to build out our collection of writing tools, and no longer worry about whether our story is good enough to survive weak characters.