Messy paints were everywhere. As part of the making of a Roald Dahl documentary I had the great honour and pleasure of meeting the illustrator Quentin Blake (please don’t use “Sir”).

Since 1978, Blake’s figures have been synonymous with something else: the characters from Roald Dahl’s children’s’ books. From Matilda to The Twits, the now-91-year-old Blake drew the ...