<P> A typical opera would start with an instrumental overture of three movements (fast - slow - fast) and then a series of recitatives containing dialogue interspersed with arias expressing the emotions of the character, this pattern only broken by the occasional duet for the leading amatory couple . The recitative was typically secco: that is, accompanied only by continuo (harpsichord and cello, sometimes supported by further bass instruments). At moments of especially violent passion secco was replaced by stromentato recitative, where the singer was accompanied by the entire body of strings . After an aria was sung, accompanied by strings and oboe (and sometimes with horns or flutes), the character usually exited the stage, encouraging the audience to applaud . This continued for three acts before concluding with an upbeat chorus, to celebrate the jubilant climax . The leading singers each expected their fair share of arias of varied mood, be they sad, angry, heroic or meditative . </P> <P> The dramaturgy of opera seria largely developed as a response to French criticism of what were often viewed as impure and corrupting librettos . As response, the Rome - based Academy of Arcadia sought to return Italian opera to what they viewed as neoclassical principles, obeying the dramatic Unities of Aristotle and replacing "immoral" plots, such as Busenello's for L'Incoronazione di Poppea, with highly moral narratives that aimed to instruct, as well as entertain . However, the often tragic endings of classical drama were rejected out of a sense of decorum: early writers of opera seria librettos such as Apostolo Zeno felt that virtue should be rewarded and shown triumphant, while the antagonists were to be put on their way to remorse . The spectacle and ballet so common in French opera were banished . </P> <P> The age of opera seria corresponded with the rise to prominence of the castrati, often prodigiously gifted male singers who had undergone castration before puberty in order to retain a high, powerful soprano or alto voice backed by decades of rigorous musical training . They were cast in heroic male roles, alongside another new breed of operatic creature, the prima donna . The rise of these star singers with formidable technical skills spurred composers to write increasingly complex vocal music, and many operas of the time were written as vehicles for specific singers . Of these the most famous is perhaps Farinelli, whose debut in 1722 was guided by Nicola Porpora . Though Farinelli did not sing for Handel, his main rival, Senesino, did . </P> <P> Opera seria acquired definitive form early during the 1720s . While Apostolo Zeno and Alessandro Scarlatti had paved the way, the genre only truly came to fruition due to Metastasio and later composers . Metastasio's career began with the serenata Gli Orti Esperidi ("The Gardens of the Hesperides"). Nicola Porpora, (much later to be Haydn's master), set the work to music, and the success was so great that the famed Roman prima donna, Marianna Bulgarelli, "La Romanina", sought out Metastasio, and took him on as her protégé . Under her wing, Metastasio produced libretto after libretto, and they were rapidly set by the greatest composers in Italy and Austria, establishing the transnational tone of opera seria: Didone abbandonata, Catone in Utica, Ezio, Alessandro nell' Indie, Semiramide riconosciuta, Siroe and Artaserse . After 1730 he was settled in Vienna and turned out more librettos for the imperial theater, until the mid-1740s: Adriano, Demetrio, Issipile, Demofoonte, Olimpiade, La clemenza di Tito, Achille in Sciro, Temistocle, Il re pastore and what he regarded as his finest libretto, Attilio Regolo . For the librettos, Metastasio and his imitators customarily drew on dramas featuring classical characters from antiquity bestowed with princely values and morality, struggling with conflicts between love, honour and duty, in elegant and ornate language that could be performed equally well as both opera and non-musical drama . On the other hand, Handel, working far outside the mainstream genre, set only a few Metastasio libretti for his London audience, preferring a greater diversity of texts . </P>

Where did early composers find plot-lines and characters for their operas