Writing a monologue for "Dracula", any tips?

I'm auditioning for our school production of Dracula next week, but for this audition we are required to write a monologue for a leading character and a supporting character. I am going to write monologues for Mina Murry and one of the Vampire Brides (if they have lines, I'll have to read the play). I have experience with acting, but I've never written a monologue before...I just need some tips, haha :) Thanks :]