So I'm writing a book where my main character is clinically diagnosed as being catatonicly depressed. If I write it without therapy will I become depressed? I've heard that sometimes those kinds of things happen to authors and they get 'stuck in' their characters lives.
As an artist I think it's a good thing, it brings honesty and truth to the writing, it brings authenticity.
However, as a human, I think anything which impacts upon your life to the degree that you are no longer able to function normally is bad - method writing included. If it's affecting you to write it - stop. Human wins out in this situation.
As has been said, you aren't likely to become depressed if you aren't already pre-deposed to it - listening to sad music and wearing black doesn't make you depressed, but if you are depressed you may listen to sad music and wear black; do you understand the difference? A depressed writer may write about depression which feeds into their condition, but the undepressed writer won't find they suddenly can't manage to get up in the morning.
Make sure you research properly (Wikipedia is not a source). Psychological disorders are complex and frequently misrepresented in books and the media. If how you "sometimes feel" is anywhere close to catatonic (or any other form of) depression, I strongly recommend you make an appointment to see you GP (if you can't, please, please, do tell somebody how you are feeling. Seek help. It's okay).
This is the best representation of depression I've ever seen, anywhere: http://hyperboleandahalf.blogspot.ie/2011/10/adventures-in-depression.html
And it's true what they say: depression is anger without the enthusiasm.
It's good to have realism in a novel and if it can be effectively conveyed. I'm not an expert on the 'inner' psychological disposition, though.
Characters can a a motive as to why they act; adverse experiences in their youth, so developing a vengeance complex (not unlike Ian Fleming's villains).
A lot of the emotions my character feels I have felt myself. I based her whole illness on how I sometimes feel so I am personally connected, but I am not catatonically depressed. I just took the feeling to the next level at some parts so it is quite scary while at the same time being very realistic which makes it even more scary... Is that bad?