They say you should never judge a book by its cover, but thankfully Steven Spielberg doesn’t pay much regard to that old cliché.

It was the fantastical front page illustration by Sidcup-born Quentin Blake that first drew the legendary director to Roald Dahl’s The BFG. And a mere 30 years later it is set to be a blockbuster hit.

Speaking at the film’s London press conference at Claridges, the man behind hits such as ET, Jurassic Park and the Indiana Jones movies recalled the first time he first encountered the book. He said: “I read it to my kids.

“I picked it up at a book store. I was more familiar with Charlie and the Chocolate Factory than I was with The BFG but it had a great illustration on the cover.

“There was a little girl and she was ‘this’ big and there was a giant with huge ears. I thought ‘this would be a good book to buy’.”

He added: “I didn’t see it as a film back then. I just saw it as a way to popularise myself with my own family.

“I read it out loud so I heard myself reading it four or five years after it was published to my first child and started to understand why it had become so popular.”

A script was eventually written – by the late Melissa Mathison, who also wrote the screenplay for ET – and Spielberg was on board to direct. But he didn’t find the actor to play his Big Friendly Giant until the first day on set of his last movie.

Last year’s Bridge of Spies saw Kent-born actor Mark Rylance pick up a best supporting actor Oscar in an altogether more serious role as a KGB spy, but even on day one Spielberg knew he had his BFG to star opposite 11-year-old newcomer Ruby Barnhill as Sophie.

He said: “It is very hard to deconstruct an intuitive tickle that I get sometimes with someone like Ruby, after looking at hundreds of other girls between eight and 11.

“Suddenly Ruby comes on the screen to audition and time stops and I dared to even imagine ‘I think I might have found her’. You can’t deconstruct that or say ‘here’s why’.”

He added: “I have known Mark and certainly met Mark in the late 1980s when I was casting Empire of the Sun. I’ve seen Mark on stage. I’m a huge admirer of his.

“He’s like a liquid actor who can fill any vessel and do practically anything, but the moment I felt he would be right for The BFG quite surprised me.

“It happened on the first day of shooting on Bridge of Spies. I just had that intuition that there was nobody better for the role.”

When Rylance was first approached with the script, however, he had no idea he was in the frame to star.

He told the press conference: “Steven asked me to read the script on the first or second day of Bridge of Spies. I didn’t realise he wanted me to read it because he wanted me to be in it. I just thought he wanted my opinion.

“I was very moved by the script, and then I read the book after that.”

For many people, both Dahl’s wildly inventive books and Spielberg’s family-friendly movies (either the ones he directed, like ET, or the scores he produced under the Amblin banner such as The Goonies and Back to the Future) were big influences growing up.

This new big-screen version of The BFG will not only bring those two together for the audiences – it did for the cast.

London actress Rebecca Hall, who plays The Queen’s lady-in-waiting Mary, first picked up The BFG when she was young.

She said: “I read it when I was a child. It was actually the first novel that I read to myself… it was definitely one of those books that I kept reading after lights out.”

She added: “I had a very special time on this. Steven’s films are so much a part of my generation’s childhood so the combination of [him] and Roald Dahl really felt for me a return to aspects of my childhood.

“There was a kind of magic and wonder and excitement that filled the whole job for me.”

The BFG (PG) is out tomorrow, Friday (July 22).