Some quotes from Dahl from the interview:

The discipline of writing:

"Your concentration is fairly intense. You're lost into this world of the story that you happen to be doing - it's terrifically demanding you know. What do I write four, four-and-a-half hours a day - so a quarter of my waking hours I am completely immersed in a dotty world of fantasy."

Writing for children vs adults:

"To my mind, there's no question that to write a children's book of comparable quality to a fine adult novel or story is more difficult - it's much more difficult to achieve the children's book."