One great way to achieve this is to act out (in your head ideally if only not to frighten the neighbour and/or domestic pets!) what your characters would do and say and ask yourself why they would do this. To literally carry out the old "luvvie" joke of asking yourself "what is my motivation in this?". That particular question is one all fiction writers should be asking of their characters. It shows up which characters are not pulling their weight (which is usually a good indication you don't actually need them in the story).
Your first reader is you. If you can't convince yourself about your characters and plot, you certainly won't convince anyone else. And I believe if a writer is enjoying their work, they will write with more confidence and readers will pick up on that. A confident writer knows their characters inside out and will write about/for them with that confidence.
This is where I can see a character biography could be useful though I tend to focus first on major traits and then fill in with some physical description. But I do know my characters and how they would act/would be likely to act. (The latter gives me some room to allow them to surprise me but even then I know enough of them to see, yes they could spring that surprise given the right circumstances!).
So are your characters convincing you? If not, what is it about them that doesn't seem quite right? And never be afraid to keep questioning. It will help keep you on track.