Do you always need the nő to specify that the person with a particular job is a woman. For instance, in the sentence "A színész nem alacsony" do I know for sure this is about a male actor? In English, even though we have the word actress, it is common for people to refer to women as actors. Is it the same in Hungarian or do you always add the nő?
Actresses are most of the time referred to as "színésznő", but it isn't incorrect to call them "színész" either.
It varies from job to job how common it is to use the female version. "Pincérnő" (waitress) and "titkárnő" (secretary) are similar to "színésznő": I've hardly if ever heard the basic form used for women. "Rendőrnő" (policewoman) is less common, I personally call female police officers "rendőr" as well. There are many jobs, mainly traditionally male occupations, where you only add "-nő" to the end when you really want or need to emphasize that you're talking about a woman.
The same, "nővér" is gender neutral (as a profession). Note that "nő" is at the beginning of the word this time, this may help you remember that it is different from the other professions. The word "nővér" also means elder sister, and in that meaning it is gender specific.