Best-selling fantasy author George R. R. Martin tried to makeGame of Thrones— and the three books that follow it in hisSong of Ice and Fire series — “unfilmable.” Says the writer: “I had worked in Hollywood myself for about 10 years, from the late ’80s to the ’90s. I’d been on the staff ofThe Twilight Zone andBeauty and the Beast. All of my first drafts tended to be too big or too expensive. I always hated the process of having to cut.” When he took up novel-writing afterward, he recalls: “I said, ‘I’m sick of this, I’m going to write something that’s as big as I want it to be, and it’s going to have a cast of characters that go into the thousands, and I’m going to have huge castles, and battles, and dragons.” Cut to this year, when HBO is prepping to premiere (on April 17) a spectacular TV version of his epic saga. How’d it happen? He talks us through executive producers David Benioff and D.B. Weiss’ small-screen adaptation of his geek-obsessing fantasy land:

ENTERTAINMENT WEEKLY: Had you considered ways to adaptGame of Thrones, even though you’d written it as “unfilmable”?

GEORGE R.R. MARTIN: David [Benioff] and Dan [Weiss] were not the first people to approach me about this. We’d had a couple of meetings with other scriptwriters, and most of them wanted to do it as a feature film. Even Tolkien took three feature films, and three Tolkien [novels] are as long as one of my books. That got me thinking, “How could it be done?” I knew it couldn’t be done as a network television series. It’s too adult. The level of sex and violence would never have gone through. I said, “The one way this could be done is by HBO.” And when I met with David and Dan, we had the same ideas. When I left, I thought I was in good hands.

What did you think of the casting?

Sean Bean [as “King’s Hand” Sir Eddard Stark] and Peter Dinklage [as the queen’s trouble-making “imp” brother, Tyrion Lannister] were our first choices. It’s amazing we were able to get them. I had a voice, though I didn’t have any veto power or anything. There were dialogues. A couple of roles, there were someinteresting discussions, but we got a great cast.

What about the kids cast? They’re such an integral part of the story.

It’s a very unusual role for children. There are a lot of child actors out there, and most of them are kinda cute and they trade on that. But we needed child actors who could actually act, who could deal with death and danger. And the kids we found are amazing. The Stark children carry a lot of the story.

This will be a big area of debate among your obsessive fans on the Web: What changes have you noticed the producers have made in your material?

There are changes, but I’m not sure how many of them are important. Like in the sets. The High Hall in the Eyrie, the one I described was long and rectangular with columns, and the one they built is round with curving staircases up. I had the Moon Door in the wall; they had the Moon Door in the floor. I know they tried, but they were not able to do the purple eyes for the Targaryens. You can do colored contact lenses, but that’s difficult for the actors and doesn’t always look right anyway. I have some great fans, but some of them get a little obsessive about things like that and are going to go crazy. But I’ve worked in Hollywood too long to worry about things like that. I’ve been on the other side of the process.

How do you think your loyal fans will react to the series?

I think 95 percent of them are going to love the show. I do think there will be a small minority who will be very upset by changes. “How come my favorite scene wasn’t in the show? How come this character’s hair is blond and not black?” But they will be a small minority. And the interesting question that no one knows the answer to is: How will the series be received by people who haven’t read the books? Because I have millions of fans, but there are many, many more millions who watch HBO, who watch television, who don’t read fantasy and certainly haven’t read my books. That’s what we’re looking forward to, hoping we please that whole new audience. There’s a tremendous amount of excitement. Every time HBO releases a new piece of information or photograph, the blogs explode with excitement.

You also wrote an episode, right?

I wrote episode 8. It is the episode where everything comes to the boil at the Landing. I titled it “The Pointy End.” It’s got the aftermath of the fight for power. I enjoyed writing it. It had been like a decade since I’d written a screenplay. Fortunately, I did remember how to do it. The biggest adjustment was the software has changed. It was easy in this case, because David and Dan are being quite faithful. It was a process of taking scenes from the book and putting them in script form. Even with ten hours [in the entire season], you can’t get in every line.

And you shot a cameo as well?

I shot in Morocco, and nothing from Morocco was used, so I’ll have to do another cameo. I was dressed in elaborate robes and I had a gigantic hat. It was more exhausting than I thought. We did many many takes and many angles. Next time I do a cameo I’m gonna do it sitting down holding a beer in my hand.

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