Out of curiosity, I've looked into the musical score used in the movie (full disclosure: I've neither seen nor heard it), and found this in Martin Bookspan's biography of Previn:
Martin Bookspan wrote:"Porgy" presented a ticklish problem to Andre. (Bookspan always calls him that.)He was not sure just how much of the film score should be Gershwin and how much Previn, hos the film score should be Gershwin and how much Previn, how much of Gershwin's own orchestration he could "rearrange" to fit the requirements of a film, but he found his answers in the original score parts, which had been measured to fit the instruments of a Broadway "pit band." Andre's requirements were something else entirely.
[Previn says,] "Gershwin had been stuck with the physical limitations of his orchestra pit. Had he been writing for a larger group he'd have written it differently, because what is orchestrated for thirty men is not necessarily going to sound good when played by one hundred... The producers wanted a rich sound and there was no way I could give it to them without reorchestrating that score."
Previn is being ignorant or disingenuous. Gershwin scored "Porgy and Bess" for a full opera house orchestra with 2 flutes (one doubling on piccolo), 2 oboes (1 doubling on English horn), 3 clarinets (2 doubling on alto saxophone), bass clarinet (doubling on tenor saxophone), bassoon, 3 horns, 3 trumpets, 2 trombones, tuba percussion, banjo, piano and strings, plus an onstage piano and band. The opera received its first production on Broadway and for that the pit band was limited to 45 or 41 players (it's unclear which), minus a clarinet, horn, trumpet, and the tuba), and the score was cut by 1/2 hour, against Gershwin's wishes; he cooperated because he wanted so badly that his opera be performed, and Broadway was the only venue where that could happen in 1930s America. Today the performances at the Met and elsewhere use Gershwin's original score, restoring most of the Broadway cuts, and Previn should have done likewise. He didn't bother.
If, as jserraglio believes, the film is an "adaptation" of Gershwin's opera, then so was the world premiere production on Broadway. The replacement in the movie of most of Gershwin's recitatives with spoken dialogue may have made it seem more like a movie musical, but the producer Samuel Goldwyn advertised the film as an "opera" pure and simple, which was what those who made the movie believed they were filming.
Previn wrote a lot of music for the soundtrack, mainly underscoring the dialogue and action. He replaced the fugal music for one of the fight scenes with his own accompaniment, and "romantically embellished 'O Lawd, I'm On My Way." But according to Pollock, Previn "relied heavily on the original scoring, sometimes as a template but often rather literally notwithstanding a general enrichment of the orchestral texture and some thoroughly reconceived orchestrations." You could say the same thing of many performances of Baroque operas before the HIP movement beginning in the 1960s put such performing editions out of bounds.
Perhaps beside the point, this point anyway, but neither Bookspan nor Previn makes anything of the fact that he was conducting a great American classic, the only American opera that has come near to entering the standard repertoire. In 1958, when the movie was made, few opera companies had done it or would do it, in America doubtless inhibited by Gershwin's requirement, enforced by his heirs, that all the African-American characters be performed by Black singers. (In Sweden a complete broadcast performance was given by an all-Swedish, all-white cast.)
A near-complete recording conducted by Lehman Engel was made by Columbia Records in the 1950s, and Lorin Maazel conducted an absolutely complete version with the Cleveland Orchestra for Decca/London in the 1970s. Now, "Porgy and Bess" is to receive its
second production at the Metropolitan Opera. If only it had been premiered at the Met instead of the Alvin Theatre, so Gershwin could have seen and heard what he wrote. But it was two decades before even one Black person was allowed to perform at the Met, and another 30 years until James Levine finally gave "Porgy and Bess" its Met premiere.
When I realized some years later who he was I was very very annoyed with myself.
And, of course, the similar fate for "West Side Story". None of these, of course, were operas but they were musicals substantially re-orchestrated and packaged for cinema audiences. Andre Previn cut his musical teeth in that environment.
