As the correspondence between Alice Rivaz and Jean-Claude Fontanet illustrates so well, friendship is more a matter of encouragement than confiding in one another, about constant trust rather than frequent contact.
A famous author was amazed by a short story written by a woman and so sent her his congratulations. She wrote back.
A poet and a novelist living in the same neighbourhood, with similar discernment and mutual admiration binding them, forged profuse correspondence and a long friendship.