I think that as a general statement -- and really with this issue, you can only talk in generalities because there's ALWAYS at least one exception to any statement -- it's fair to characterize opera as being more concerned with music, while musical theatre is more concerned with words. While, yes, you do have exceptions like the Mozart operas mentioned earlier, Boito's brilliant librettos for Verdi (Otello and Falstaff) and Ponchielli (La Gioconda), several of Wagner's operas and some of the librettos written for the verismo operas of the early 20th century (Tosca, Butterfly, Pagliacci), to name a few, in most operas (especially Italian bel canto and buffo ones of the 19th century) coherent plot, finely wrought storytelling within the lyric, and logical character development were all clearly quite secondary to the expression of beautiful sound. In fact several very popular and famous operas really don't make all that much sense (I can recall a very funny article from Opera News several years ago in which they asked several major divas who had been great Leonoras in their prime to explain the plot of Il Trovatore -- most just laughed and said that they couldn't possibly untangle all of those plot twists, despite having sung the role dozens of times; the opera's on-going popularity is due entirely to Verdi's incomparable score, not its libretto which few people, including Italian singers who have studied and performed it, can make sense of).
Look at the the great bel canto operas of the early to mid 19th century by composers like Bellini and Donizetti -- they featured some of the most gloriously lyrical passages in the history of music ("bel canto" literally means "beautiful singing"). They were written to fully exploit singers' florid passagework and legato line, not their diction or ability to convey some deep meaning in the words (there wasn't any). They were showpieces for divas well into the 20th century, but went out of favor for a time after versimo emerged as the popular choice due to verismo's emphasis on character and storytelling. It got to the point that many major conductors and opera houses thought the form beneath them and stopped booking those operas because they weren't considered "serious" music. It wasn't until Maria Callas (one of the most brilliant singing actress in the history of opera who had absolutely impeccable diction) single-handedly caused a bel canto revolution when she had several of them revived (Lucia, Puritani, La Sonnambula, Anna Bolena) and brought so much intensity and expression and psychological depth to her interpretations (which she achieved by emphasizing the meaning of the words and emotions over simply trying to make a beautiful sound) that she caused many to re-evaluate the merits of these operas and as a result they were brought back into to repertoire of several major opera houses.
It's ironic that the mantle of the bel canto revolution was then passed on to Joan Sutherland, who continued the process of unearthing these long-forgotten 19th century operas and having them revived as vehicles for herself. Unlike Callas, Sutherland was no actress and much like the divas of the past who had initially caused these operas to go into disfavor to begin with, she was far more concerned about using this music as an excuse to show off her impressive coloratura technique and the vocal fireworks she could generate, than about creating a complex character and paying careful attention to words to tell the story -- with possibly the mushiest diction of any singer to have a major international career, it was often difficult to determine even what language Dame Joan was caterwauling in, let alone what she was ever actually saying. But audiences love their fireworks and she could fire them off with the best of them, so she became one of the most popular singers of the latter half of the 20th century.
Generally speaking (very general), musical theatre performers are better trained and more adept at lyrical interpretation than opera singers are. Part of that is due to the incredible technical demands in opera which can in many cases make it difficult for a singer to focus on anything other than hitting and sustaining the proper notes at the proper time. Opera composers tend to write much longer lines covering a far greater range than most musical theatre composers (and also write much heavier and denser orchestral accompaniment that can make it difficult even to be heard), which makes conveying any meaning to the lyrics a real challenge. Contrast that with, say, Sondheim, for whom the lyric always takes precedence over anything else and writes his scores to compliment rather than to compete with singers (as well as having his orchestrators keep arrangements relatively spare -- other than in, say, choral moments -- so that the lyrics are always easily intelligible to the audience). It's always interesting to hear Sondheim shows done by opera companies. Oftentimes, there's a mix of opera singers and a few Broadway singers and while the Broadway people typically are better at lyric interpretation and sacrificing vocal beauty for lyric clarity, the opera singers have a tendency to extend phrases and vocal passages to make a more beautiful sound and generally aren't as precise or nuanced in their acting choices. Again, not ALWAYS, but generally, that's the difference you hear between the two.
Now, as for the complaints about opera "singing style," understand that that's a necessary evil due to the tremendous demands of the music. As orchestras expanded (beginning with Beethoven in the early 19th century and continuing with Wagner decades later), operas got longer (also due largely to Wagner) and opera houses got larger, it became necessary for singers to develop a technique which would allow them to be heard clearly and precisely over and through 80+ musicians playing in alt, for hours at a time, without damaging their voices in 3000+ seat opera houses with no amplification. That "style" which employs the diaphragm to push the sound through the vocal chords with as little stress as possible is the only way a singer can possibly perform 4+ hours in a 3,800 seat house like the Met and not hurt themselves and their instruments. It's not like singing on Broadway where every one is mic'ed up the wazoo to the point where you can literally whisper and be heard in the last row of the balcony (in a house that on Broadway has at most only about 1800 seats). Amplification allows for a completely different, more "natural" singing style (and it should be noted how different even Broadway singing styles were before the introduction of mics -- belters like Merman and Jolson and quasi-opera types like Helen Morgan, Joan Roberts, Alfred Drake and John Raitt also out of necessity had singing styles that bear little resemblance to the way most Broadway singers sound today). Now, mind you, I'm not saying that you shouldn't dislike a more operatic singing style if it doesn't suit your taste -- I'm just pointing out that under such demanding circumstances, it's not really a choice.
"What a story........ everything but the bloodhounds snappin' at her rear end." -- Birdie
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"The Devil Be Hittin' Me" -- Whitney
Updated On: 3/17/07 at 04:00 PM