I don't do it. It's a waste of time and I'll never look at the form again. Really, who cares what your hero wanted, but didn't get for his last birthday? What their favorite food is, What they dreamed last night, unless it drives the story or is, at least, relevant. Granted, it's a good way to get to know your character, but I prefer seeing those things as they unfold in the story.
I've tried to do interviews. When I first heard about the concept, I got excited. Then I saw the questions: What is your biggest strength, Achilles heel. These are questions that when asked of me, I hyperventilate. When I tried answering for my character, I had the same reaction.
Things that will always come up at one time or another in a story: What kind of car does he/she drive, quirks, idiosyncratic, friends, acquaintances, occupation. How they look, the clothes they wear, are they wealthy or poor, etc. Things that define the character for the purpose of the story.
What matters most, in my humble opinion, is what happened to get them to where they are. Why they react the way they do. Did they have mommy or daddy issues, was there unfinished business that they need to resolve.
How much of the hero is the author? I'd say we, as authors, give some of our views to our characters and we, or I should say, I, bestow on them traits I would like to have myself. The most fun characters I write are those with quirky personalities. But that's another subject.
If interviews help you get to know your characters, then that's great. Some people love to fill out forms. That's not me. I feel like I'm going to break out in hives. I wish I could be organized like that, but organization is something my characters might have, that's not me. If it were, my novel would have, not only a character interview, but an outline, a pre-synopsis, proper formating from the get-go, correct punctuation, T's crossed, i's dotted.
Suffice it to say, we/I end up deferring to type.
Have a great weekend and God bless