I guess you could observe them as two different takes on the same script, particularly in the case of Romeo and Juliet. Both the play and the film present the same script in different ways. Though thinking of the various Star Wars games that have come out, they've mainly been successful due to the fact that they work within a canon of other material without appropriating directly from the films. Regardless, I don't think that the games that are based directly on the films (and vice versa)are particularly good; for instance, by creating a game sequel to Scarface the film is undermined. The purpose of the gangster film is to show it as a tragic destruction of a character, by reviving Tony from his demise at the end of the film turns it from a socially ordered film into a gratutious killfest. I don't know whether the game will attempt to end with a dramatic death at the finale, but the fact that they resurrect Tony at the start from such a dramatic death in the film means that any future death is immediately called into question. I can't see a way in which the game can present itself without potentially damaging the credibility of the film. Perhaps it would be better for game designers to be able to rely on original material, rather than appropriate scripts from film texts, but I suppose there would be less financial input for games that aren't based on "proven" films.

The CFP also mentions sequels and adaptations, so if you wanted to write for the issue you could. (and I think it would be a cool idea) I would be tempted to say that the videogame versions of things like Scarface are more adaptations, since it's a cross-media event. I would also call movies of Romeo and Juliet adaptations, for the same reasons.