I'm 38 and read this probably between 1993-1997.... when I was between 8-12 years old. I don't think it was a new book at that time.
It was about a young boy and girl who auditioned for a local play where they thought the lead actor was Dracula and tried to solve the mystery of whether he was or wasn't a vampire.
The girl got the part because she had a better stage scream. I remember the young girl saying "it's my peasant girl costume" but that is about it. I believe they only suspected him, and he turned out to be a normal person.