Call someone "a ___ freak" if they're really interested in a specific topic. Some examples include:
I admit that I used to be somewhat of a comic book freak back then.
Since when did you become such a cooking freak?
When you call someone "a ___ freak", it means that you think they spend too much time thinking about that topic. The word "freak" has a negative meaning; it means someone who is not normal and has a strange appearance or personality. However, "a ___ freak" doesn't sound that negative. It makes people think of someone who is really into a hobby.
You can also call someone "a ___ nerd":
She's such a music nerd.
But "a ___ freak" is someone who's really emotionally involved in their hobby. "A ___ nerd" is someone who knows a lot about their topic, but may not be as energetic as "a ___ freak".