I have a tendency to favor writing about one of the characters in my story more than the others. I understand this character better than any other, and my writing comes faster when he is the character whose point of view I'm conveying. The more I write this character, the more developed he becomes and the more I want to write him... this gives way to a possible chicken-and-the-egg situation where I'm feeding the problem I'm trying to prevent. It wouldn't be an option for me to simply make him the main character of the story, because it's significant that he remain slightly less of a focal point. So I wonder if anyone else has been so drawn to one character in a story that it can get in the way of writing the others. I suppose I'm looking for personal experiences more than advice, but any comments are appreciated.
You might want to consider writing the story from that character's POV. If you find him/her that interesting it is likely others will also. That has personally never happened to me, but if it does I will probably rewrite it from that character's POV.
This isn't so much of a problem for me because I'm used to writing numerous well-developed characters in my stories. *shrug* But perhaps if the secondary character is speaking to you so much, you should try another story that's theirs alone--"exorcise" them from this story in a way--and then try again to focus more on the MC of this story.
If there is one character overshadowing the others, it's probably because you've given that character a lot of attention and neglected the others. I know because I've been there. My current novel is all in first person with different characters narrating various chapters. There are about six narrating characters, but only two of them are true main characters. It's tough because one of them is a lot like me (and therefore very easy to write), while the other is almost a polar opposite. (The other three narrating characters fall somewhere in the middle; some I like, some I don't.) I found a technique that seems to be working well. I try to get myself in the character's head and write the story as I think he would see it. Then I read through it all and mark things that seem out of character. (There are a LOT of them!) Then I write again until it seems right. When I first started, I didn't have a full grasp on why this character is the way he is, but the more I write in his voice, the easier it gets. Some of the things I had planned for him don't seem right anymore. His role in the story is shifting toward something more realistic. But this only comes with time. Practice. That's the only advice I can give you.