One of the longest running issues fans have had with the Harry Potter films is how they’ve been adapted from J.K. Rowling’s books.

In this fascinating bit from Rowling and Dan Radcliffe’s one-on-one conversation found on the Deathly Hallows – Part 2 DVD/Blu-ray, the pair discuss adaptation and why it would never be perfect. Rowling also reveals a couple of times she put her foot down when she read a script, including a conversation with producer David Heyman about Harry’s green eyes.

Radcliffe: What was lovely for us, and I think I speak for everyone on set. You were around a lot at the beginning and early on. And as you kind of saw that we weren’t going massively off script you backed off. Was that hard?

Rowling: It was easy and a relief. I was around a lot early on. I wanted the Great Hall to look right, I wanted Diagon Alley to look right. There were details that I saw so clearly in my mind. I knew I could help. I knew I could genuinely help and make it right for the readers. I felt a huge protectiveness and loyalty to the readership. Once I knew it was running, it was fine.

I have to say, inevitably you have to depart from the strict storyline of the books. The books are simply too long to make into very faithful films. I could think of many places it works just beautifully.

It didn’t have to be a word for word transcription.

Radcliffe: I do sometimes think, if we did make a 6 hour Harry Potter film there would be an audience.

Rowling: And they’d still be complaining that things were wrong. And they’d still want the directors cut. So let’s not even go down that route.

Radcliffe: Is there anything – talking about things being cut out – is there anything we’ve cut that you were upset about? And is there anything we put in that weren’t in the book that you thought were great? Because I remember with Alfonso and the Dementors..

Rowling: No, I remember exactly what it was with Alfonso. First of all, on the Dementor point I thought he did those beautifully. I loved the fact that they created the visceral dislike for the Dementors. I love what he did for the Dementors. What it was, there was something in the script. Alfonso really wanted to get music into the film. He put the choir in, which I loved. But at one point he had this bizarre scene where Flitwick was conducting and there were miniature people in an orchestra inside something. And I just, you see, this is my geekiness, I said to him, “But why?” I know it’s visually exciting, but part of what fans enjoyed about the literary world is there was a logic that underpinned it. There was always logic to the magic no matter however strange it became. I know it’s intriguing to go through the mouth of whatever it was and see these little people, but why? Why have they done it? For you to film it! That’s just what it feels like. You know, normally with the magic there’s a point. So we had a bit of a discussion about that.

Sometimes I’d dig my heels in about the funniest things. I’d say, “Yeah change the costume. Yeah it can happen in that city instead of that city. And all of a sudden, I’d say, ‘But they wouldn’t do that spell. Why would they do that there? So I think sometimes I confused people. But I also remember, right back at the beginning when you were cast I remember David Heyman calling me up and saying, “We’ve tried green contact lenses. We can digitally alter his eye color in post production. How important is it that his eyes are green?”

Radcliffe: That I will thank you for.

Rowling: And I said, um, “The only really important thing is that his eyes look like his mother’s eyes. So if you’re casting Lily, there needs to be a resemblance, but they don’t absolutely have to be green.” “Oh thank Christ,” he said. Were the lenses that awful?

Radcliffe: There is a very small percentage of people apparently who have a very extreme reaction to contact lenses. And I was one of them.

Rowling: You poor thing. I feel really bad.

Radcliffe: No, don’t worry! It was 10 years ago. It was really fine.

Yesterday we shared a part of the conversation where Rowling thought Radcliffe, Rupert Grint, and Emma Watson were too attractive for the roles.