Ian McKellen wrote the screenplay while touring the play. Originally he thought it would be a television production, but he soon realized that the large production he envisaged would require the budget of a feature film. Still on tour, he began to consider screen actors he met for roles in the film, including Patrick Stewart, Danny DeVito, and Meryl Streep.
Ian McKellen enjoyed acting with Robert Downey Jr. in Restoration, and asked him to play the part of Rivers, expecting him to turn the role down as too small. To McKellen's surprise, Downey immediately cleared his diary and and took the part.
Ian McKellen made Clarence a photographer to amplify the fact that he never noticed anything. He was merely seeing the events, not watching them. Therefore, he never realized how evil Richard actually is.
Since the film is set in the alternate 20th-century, Ian McKellen had some trouble of setting the location for Act 3 Scene 1, where the Prince of Wales meets Richard before being sent to the Tower. Someone suggested that the royals usually arrive in London by rail. The scene was eventually known to everyone in the film as the 'Victoria Station' scene.
The trivia items below may give away important plot points.
Only about half of William Shakespeare's original text is used. Sometimes details were changed, e.g. Lord Rivers (Robert Downey Jr.) was killed much later in the play than he is in the movie and when Queen Elizabeth (Annette Bening) receives the news of his death, the same messenger in the play only told her of his indictment on Richard's and Buckingham's orders. Queen Elizabeth and the Duchess of York have larger roles then in the play. An important character in the play is Queen Margaret, widow of Henry VI, who does not appear in the film. Many of her lines are given to the Duchess of York, although these lines do not suit that character.
The play did not explain how Lady Anne meets her fate. Ian McKellen based the idea of how she dies on two elements: a line in which Queen Elizabeth calls Richard a 'bottled spider'; McKellen also based it on stage actor Robert Helpmann performing in front of backcloth painted with spider webs. In the finished film, a model spider was superimposed on a static shot of Kristin Scott Thomas.
In the play, Queen Elizabeth simply disappears after Act 4 Scene 4, so a scene in Richmond's headquarters where Richmond marries the princess was added to give a proper resolution for the Queen, being the principal survivor of the film. The Archbishop's lines in that scene were taken from Richmond's lines at the end of the play.