The answer to this question, for most people, will probably be a generic one, not necessarily based on specific differences in plot, character, and theme between play and movie, but on the distinct qualities any film will have vis-à-vis a play upon which it's based.

Film is by nature a more "realistic" medium than the stage-play. An audience watching a play in the theater knows, of course, that this is a performance and that the actors are playing roles. There is a self-conscious quality to the theater, a deliberate kind of artifice. This is not said in a negative way, but simply as a description of what makes the theater what it is. It is also precisely the feature for which those of us who enjoy watching and reading plays value the theater.

In film, on the other hand, the intention of the director is usually to avoid artifice and to make the presentation of a drama as naturalistic as possible. The action of a film, in some sense, is made to appear as if it's something that really happened and has been caught on camera. The audience, of course, know that they're just watching a movie, but it seems as if it could be real in a way the theater can't accomplish—and doesn't try to do. And ironically, there are instances in which a low-budget film can seem especially real, because it's less artful than a high-class production and can appear like something that really took place and just happened to be filmed with a home-movie camera.

In A Soldier's Story the exterior scenes showing the death of Waters perhaps bring home the reality of the situation in a special way possible only in film. The physical details of the army barracks and the army base as a whole have a similar, gritty appearance that the stage play cannot recreate. But given that the film follows A Soldier's Play closely in plot and theme, these "advantages" of film are minor considerations. One can just as easily assert that in the theater the physical closeness of the actors to the audience gives another kind of realism or emotional immediacy lacking in film. In any event, both versions of Charles Fuller's story are masterpieces.