I've struggled with these issues as well, not knowing my characters or story well enough before I start telling it. The character interview you describe is a great jumping off point to figuring out the lay of the land.

But I've found many times that answers to these questions in the early days can seem arbitrary. I find myself coming up with stuff that may or may not be useful in the story itself. Does it matter that my character hated his mother? What if this fact never appears in the story? Then does it exist? And without the context of what I'm trying to do with the story, how am I to know if my character does actually hate his mother? Flip a coin? Trust my unconscious? Chicken or the egg?

What I find interesting though is to do a fast characterization, figure out what the character wants and needs, figure out the relationships, and then write the first draft. In the telling of the story, I'll find that stuff I didn't know about my character will suddenly start revealing itself. Sometimes it'll happen on it's own, and other times I'll decide the character needs to hate his mother to help drive a piece of the story. The information of characterization that is important to know will make itself known in the course of writing the script.

What's interesting is to do the character interview before you write your first draft and then do another after. Notice how many details changed. Hopefully in writing we're flexible enough to allow our characters to reveal themselves in the story, rather than semi-abitrarily creating a history and forcing the story to conform to that history.

In any case, for m,e it feels like character details and pre-plotting, to a certain extent, has to be a fluid, ever changing experience, informed by and informing your story as it develops.

I used to have horrible ACT 2 issues as well until I read this line:

ACT 2 is when you deliver on the promise of the trailer.

I'm not sure who wrote that... the late Blake Snyder perhaps? Maybe it was you, Scott, I don't recall...

But that ended all Act 2 tribulations. It was the first time the 60 or so pages located in the heart of the script made complete sense to me.

Real Thanks Mr Scott.

I'm doing this method of characterization.

The Heath Ledger Method.

I sit down and make my character write their diaries. They tell me what they feel and how was their day and most important what's missing for them.

I decided that I'm gonna write a whole scrapbook for each of the main characters.

Real thanks for your advice Mr SCott.

Best Teacher Ever

As a tool for that kind of thing, I recommend Movie Outline. It has a very comprehensive section for everything character.

Admittedly I've abstracted my character creation methods to be based on position (ally, mentor, etc) and dialog type. Are they sarcastic? Demanding? Inhibiting?

But I do whole-heartedly agree that at least 1 week of research and outlining. WikiPedia is a screenwriter's friend.

I concentrate more on external factors though rather than personality, etc. I look at the job specifics since you can make scenes where those traits\abilities save them or someone else.

I also look at education because that again can provide "scene fodder." If a person is a PhD, they can pull "McGuyvers." If they're a vice cop they can smell drugs.
If a computer nerd they can hack into the BG's (bad guy's) computer.

I think this may be (if there even is such a thing) the single most important aspect of screenwriting--of any kind of writing.

It takes me a LONG time to get a story. To have it. And when I don't know the story, then I am not confident, and I brutally judge EVERY WORD I WRITE--be it a scene slug or dialogue, or whatever.

But when I KNOW my story, why the scene I'm writing MUST occur, then I just tell that little demon on my shoulder, "shut up, YOU don't know the story. Just watch."

"Going into the story" is the point of it all I think.