The technique in opera in which the characters sing through lines that would usually be spoken is called

Recitative

Recitative is a style of delivery where natural inflections of dialogue are duplicated adopting free vocal rhythms. "Recitativo" is a monodony passage more like a speech than a formal musical composition. The two principal varieties of Recitative are Recitativo Secco which is characterized by free rhythm. and Recitativo Stromentato which is distinguished by stricter rhythm.