If I asked you which is your favourite book it is likely you'll name one where its characters have stayed long in your mind after you came to the last page. Emma in Emma Bovary, Mrs Ramsey in To the Lighthouse are just two women with whom I've shared their lives.
As a writer intent on creating strong characters you need to know them through and through if they are going to come alive on the page.Spend some time living with them, in your imagination, 'talk' to them, preferably not in public or you might get some funny looks.
If brain storming is your thing, sit yourself down with pen and paper and sketch them out as fully as you can. This way you will know how they think, feel and act which will influence how you write about them.