In early opera since Jacopo Peri's Euridice (1600), now considered the first of its genre (1561-1633), there was a smooth transition between long narrative (recitative) and short melodic (arioso, songlike, dance) sections. In the course of the century this contrast became more and more pronounced and reached its climax in the opera seria, in which the plot-bearing sections (secco-recitatives) and the contemplative-commentary, sometimes philosophical-teaching arias are clearly separated from each other. The recitative dispenses with the repetition of words and consists primarily of the dialogues or monologues that advance the plot. It is thus a recitative in which the words are foregrounded. A well-composed recitative takes into account in its musical text the natural singing of the language, that is, the rhythm and pitches should correspond to the speech-ductus of (in this case) Italian. The recitative is accompanied by basso continuo: chordal instruments (usually the harpsichord and/or an instrument of the lute family) and a violoncello. A da capo aria is the conclusion or interlude of a scene: the action comes to a halt and an actor is given space to comment on the state of the drama and to express personal feelings, according to the doctrine of affects. This can be a direct reference to an event or a comparison with a life situation familiar to many people in the form of a "parable aria". Such an aria can often last up to 7 or 8 minutes, consists of two or three verses whose set pieces are repeated several times, and has the form A-B-A. The first section introduces a theme, usually in direct response to the final action section in the preceding recitative. The B section usually highlights a different aspect of the theme addressed or is addressed to a different person. As a result, it may differ fundamentally from the A section in expression and timing. In the repetition of the A section, the aim is now to present the previously expressed feelings of the person acting in a more forceful and determined manner. This demands a maximum of affect-enhancing ornamentation from the vocal part. After the aria, which is usually accompanied by strings, strings and oboes, and sometimes horns or flutes, the person usually leaves the stage ("exit aria") and the audience applauds. A typical opera seria consists of about 30 such numbers, not counting the recitatives.
The sequence and constant alternation of recitative and aria is preceded by an overture in the form "slow-fast (usually a fugato)-slow", which, however, has no relation to the plot. Curiously, already in the early 18th century this form of the French overture prevailed in Italian opera against the actual Italian Sinfonia (fast-slow-fast). The pattern of the sequence of the movement-pair seccorecitative-aria is sometimes interrupted only by an occasional duet of the main love couple, or this stands effectively at the end of an act. In moments of particularly violent emotion, a recitativo accompagnato or recitativo stromentato is preferred to a "secco," in which the singers are usually accompanied by the strings. The plot of an opera seria is usually divided into three acts. Its actors are kings, nobles, gods and heroes of ancient mythology (e.g. Handel: Deidamia, Hasse: Didone abbandonata). Here the rules of strict, closed drama apply. In one main and sometimes many subplots, conflicts are first developed, then brought to a head by intrigues or deceptions, only to be resolved again at the end. The constellation of characters is also standardized: in such an opera there are about six to seven people who can be seen standing in the first and second rows: the prima donna and the primo uomo (a castrato) as well as usually a tenor for parts such as a father or a king in the first group, a seconda donna and a secondo uomo as well as one or two other performers for smaller roles (bass, soprano). In terms of character, the figures are not psychologically complex and sensitive subjects, but representations of one type of person: they either represent virtue, baseness or oscillate between both poles and ultimately decide in favour of virtue. The guiding principles of their actions are always love, honour, duty or, on the other hand, power, fame, property. They are not (in the sense of the Enlightenment) freely acting persons, but mostly subject to the will of the gods or to fate. It was only Handel who, through the illustrative power of his music, developed his characters into persons who also take on a development in the course of the drama, and whose arias illuminate the different sides of these personalities. The few choruses are not in the sense of the ancient drama, but represent, as it were, a multiplied individual being and not a mass of sentient individuals.
The dramaturgy of opera seria largely follows the ideas of the Rome-based Accademia dell'Arcadia. It attempted to return to the classical principles of virtue (Aristotle), taking as its basis highly moral poems intended to instruct but also to entertain. Unlike classical drama, opera seria librettists rejected the tragic ending out of a sense of propriety: virtue should be rewarded, and so there is always a happy ending with a jubilant final chorus. Shortly before the end of the often very entangled plot, rich in subplots, a fit of magnanimity is imposed on the protagonists by the poet, in which they forgive all their adversaries within seconds, or a deus ex machina intervenes to make the lieto fine possible. Here it is Handel again who partially breaks with this convention (having, of course, often served it), allowing, for example, a tragic event in his Tamerlano and setting a reflective final chorus.
Each leading singer at the time was allowed to expect his fair share of sad, angry, heroic or meditative arias. About this, the contemporary poet and librettist Carlo Goldoni wrote: "The three main characters in the plot should each sing five arias; two in the first act, two in the second and one in the third. The second actress and the second soprano may have only three, and the supporting roles must be content with a single aria, or at most two. The writer of the text must supply the composer with the various shades from which the chiaroscuro of the music will arise, and he must take care that two solemn arias do not follow each other directly. The bravura arias, the action arias, the subordinate arias, the minuets and rondeaus must be distributed with the same care. Above all, he must avoid assigning affect arias, bravura arias, or rondeaus to the secondary roles."