It is a tremendous thrill for me to make this offer

In 1989, director Steven Spielberg made the romantic fantasy drama Always, a film that is generally considered a lesser entry in his filmography. Like Spielberg's recently released West Side Story, Always is a remake of a classic Hollywood film, i.e. Victor Fleming's A Guy Named Joe (1943), which was one of Spielberg's childhood favourites and one of the films that had inspired him to become a film director. A Guy Named Joe is about an airforce pilot (Spencer Tracy) who is killed in combat during World War II and is sent back to earth by a Heavenly "General" (Lionel Barrymore) on a new mission. For his remake Spielberg changed the setting of World War II to a modern setting of aerial firefighting. Richard Dreyfuss was cast in Tracy's role and in Barrymore's role Spielberg wanted Sean O'Connory but he was unavailable. The director then decided to replace the "General" with a female angel ("Hap") and approached Audrey Hepburn to play her.

Seen below is the letter Spielberg wrote to Audrey on 14 July 1989, offering her the part of "Hap". In his book Audrey at Home (2015), Luca Dotti recalled his mother's reaction after receiving the letter: "Years before, watching E.T. at a movie theater in Rome, my mother, extremely moved and squeezing my hand, whispered to me, "Luca, this man is a genius." Now that genius was offering her a role. When I asked her what the part was, she replied, "But it doesn't matter! Do you realize he actually wants me?" (Audrey was given a small role, making a cameo appearance.)

Probably best remembered for being Audrey's last film, Always was neither a critical nor commercial success. Still, it was a film both Spielberg and Audrey had enjoyed making. Spielberg said that one of the greatest thrills of his life was working with Audrey, while Audrey commented in an interview with Larry King: "I loved it, and I wouldn’t mind if he asked me again, like next summer. I’d be right back. I had really one of the best times of my life." 

At the time Audrey was quite busy with her work as Goodwill Ambassador for Unicef and her $1 million salary for the film she donated to the organisation.

Source: Christie's