I am new at this so I am not the best person to answer but I will put in my two cents anyway.
I didn't put hardly any research into my first novel, still unfinished by the way.
I am taking a novel writing class and the next few weeks is world building. This weeks assignment is all about research, the what and how of it. I try and write about what I know, so not much research is required. However, for my class I am breaking new ground and will get to see just how much research is needed.
As for reading similar books, yes and no. I have skimmed the titles and synopsis's of similar books to make sure there is no unintentional plagiarism but I do not obsess over it. I figure if I came up with the idea, how could I be duplicating someone else? I know that sounds silly but if you give ten people the same idea, you will get ten different stories.

Well, when I have a story idea first I work on that - conflict, characters, environment, situation, what I want or think the outcome will be -- and I note down my questions when I find a hole in my knowledge. I guess the first step is, Figure out what you DON'T know. Then go research.

When I research I am looking for info that supports my story, contradicts it, deepens it, takes the characters in a different or more truthful direction, sharpens the conflict, ups the stakes. I keep writing while I am researching.

I do look through and often read novels on the same theme or are otherwise similar, because in selling the book you'll have to cite comparable titles. When I read these other novels, I am really looking for how this writer handled the material, and is it effective?

There is a fine line a fiction writer will walk and a decision to make - when to make the leap that is true to the story but departs from the research. I heard a successful contemporary novelist say at a conference talk, he builds a sense of plausibility in the reader by setting up the story thus: "Fact, fact, fact, fact, fact, fact, fact, fact, fact, COMPLETE B--S--T, fact, fact, fact."

And you know, a story wouldn't be interesting if there were no surprises. ;-)

Like Joy, I research and write at the same time. The amount of research I do depends on the individual piece. Of the four stories I've got forthcoming, the shortest one in the bunch required no research at all. Another (originally published in 1988, forthcoming in an anthology), was inspired by articles I read on spiders and on cosmic strings, and led me to do research on Greek myth and on mitochondrial DNA. The third required minimal research, consisting mainly of reading teenage blogs and going back through my own high school diary. The fourth, on the high research end of the spectrum, had me going through more references in Greek myth (I like those old gods), body mutilation art, raptor rehabilitation, and Munchausen's Syndrome by Proxy.

In each case, I ask myself: How much detail must I include to make this believable? If I don't know the specifics of the details, where can I find them? Often as I'm writing, the story will lead me to a place where I stop and look for info (I don't use placeholders unless I absolutely have to). Writing speculative fiction lets me play around with "facts" -- I usually start with the characters and their situations and then look for ways to put them through hell. :-)

I rarely look at similar books in the genre while I'm doing the writing, because the thought of inadvertently duplicating someone else's idea scares the crap outta me. When I'm living inside a story I am much more sensitive to external cues and similarities than I otherwise would be, so even a different plot line with similar elements has the potential to make me curl up into a paralyzed ball. I've got a novel draft in the deep freeze that shares various elements with a book by a big name author. When I learned about this other book (which was published several years after I'd started my project), I bought it immediately when it came out (in hardcover, which I rarely buy), and I shoved it in a drawer and didn't peek at it until after I'd finished writing my own draft. My story and the other one are considerably different, and I can change some of my details (there are similarities in character name, scientific discipline and ensuing crisis, even eye color), but I go after my own vision first and then tweak it if I feel an uncomfortable overlap with something else. In this case, my story (which I started writing in 2000, while working multiple shifts) is a very rough draft and needs a major overhaul anyway.

Kind of in line with the research question, I'm drafting a story in which my protagonist is a working-class Hispanic woman and in which the theme deals with environmental racism. I am fully and at times painfully aware that I am a middle-class Jewish woman writing considerably outside my own culture -- and while I am transplanting some details from my time of living in what the Boston Globe once called the city's most ethnically diverse neighborhood, on a certain basic level I really don't have a clue.

As a writer and as someone trying to understand and empathize, not having a clue bothers the heck out of me. Half the time I wonder if I should be attempting this story at all, from an ethical standpoint. On the one hand, I'm responding to its vision. On the other hand, I experience the vision from within my own limited and ignorant perspective. On the third hand, no one group is monolithic, but contains its own degrees of diversity. Has anyone out there wrestled with this kind of cultural divide?

I do loads of research---I really enjoy it! It's less work than actually writing and it takes me places I haven't been before. Oddly enough, I find my best stuff comes when I *don't* write what I supposedly know, but when I decide I need some knowledge I don't already have for a piece, and go looking for it. Thank goodness for libraries, encyclopedias, Wiki, etc.

As for similar productions, it's tough to navigate through a field without overlapping it at some point. I actually don't read as much fantastic literature now that I'm writing my own, so maybe I'm avoiding it on some level for that reason, but you can really only pull that off if you've gotten your basic grounding in that genre or category first.