The "Bird and Baby" pub was actually "The Eagle and Child" pub, where C.S. Lewis was a regular.
Nigel Hawthorne, who played the role on stage, was replaced in this film version by Anthony Hopkins because Hawthorne was still almost unknown internationally.
Richard Attenborough replaced Sydney Pollack as director. When he came on board, he had Anthony Hopkins and Debra Winger in mind to play the lead roles.
Barbra Streisand was asked to direct and star, but declined so she could pursue The Normal Heart (which never got made).
Graham Sinclair (uncredited) is the voice of the taxi driver in the country.
In one of the scenes in which Douglas Gresham is reading in bed, you can see he is reading The Hobbit by J.R.R. Tolkien. Tolkien was a good friend and colleague of C.S. Lewis.
Douglas Gresham became the heir to C.S. Lewis's literary estates, and is one of the producers of the film series The Chronicles of Narnia.
Anthony Hopkins prefers to memorize lines on his own, while Debra Winger prefers extensive on-set rehearsals. To accommodate Winger, Richard Attenborough would rehearse with her, reading Hopkins's lines.