And if the story I am writing is interesting but I have.... totally no knowledge of, I research by interviewing people [...] I do the first method the most, because they are experienced people in their areas, so you can surely extract accurate information.
! In which way, how many people, who and about what do you interview? I could understand asking a professor/teacher about something they specialise in, but unless you do it well and the topic is entirely objective, interviews can give you really non-representative answers.
Firstly, I make sure about my teacher's background (my teachers are my initial target; they're easy to reach and they're pretty friendly too). Since I mostly research about medical conditions, I am interviewing my biology teacher. But I should know first whether she's just book-taught or she had a degree in biology itself. I knew mine took a degree; she told the entire class herself, and my botany teacher knows as well. And then, I first analyze what I want from her, if she knows it, and if she can clearly answer my questions. I once was interested in the disease she discussed from us (which is sickle-cell anemia), and so I'd think on the information I want, and what I only want. I try to be as specific as possible. And then I make on for the questions. I try not to make the questions annoying (like too many follow-up questions).
And for the number of people, it actually depends on the people available around me. Many is good, for confirmation. But I don't really know the other biology teacher, so after the interview I sift through books and do a little Wikipedia-ing, to check if they are similar (and yeah, I don't really trust Wikipedia, although I got my homework there several times). If all are similar, then celebrate! If it isn't, you just got trolled (by either Wikipedia or your teacher).
The good thing with interviews is that if it's done right, you get every little detail possible. For example, if you know the Mexican novel "Like Water for Chocolate", in the 6th chapter, there was a part there where a character lectured what is phosphorus, who discovered it, how was it discovered. Yes, you can go that detailed.
I try my best to make interviews frequent, but then again, they can get annoying as well XD So if you ever plan, do it all in one shot, except if you hate your teacher and you want to annoy him/her.