"Conversation piece" was one of Hogarth's contributions to enlargement of the scope of painting in 18th century England. The idea refers to a painting of an informal group, which was not in itself new, nor the invention of Hogarth alone. The small-scale picture of family or friends in their habitual routines or diversion became popular early in George II's reign under the influence of Flemish and Dutch prototypes, to which were added the influence of pastorals and scenes from Italian Comedy and their groups of actors.
Hogarth's influence appropriated the conversation piece for the English, exhibiting his dislike for formal portraiture and what he contemptuously called "phizmongering." Pursuing the taste adjusted to the middle-class consciousness led him to paint many individual portraits on a scale approximating life-size of the common man and woman, and these portraits are sometimes considered to be his best work.
The small conversation piece was another escape from the boredom of conventional portraiture. The 'puppets' -- the models or characters in his paintings -- could be arranged in a composition to his liking, and such agreeable side activities were involved as the still life of objects in an interior or the painting of a landscape backcloth for an open-air group. Taking advantage of the growing vogue for such small works in the 1720s, and under the necessity of earning more money than the craft of engraving could provide, after his runaway marriage to Sir James Thornhill's daughter in 1729, Hogarth quickly became the leading exponent of the genre.
Joseph Highmore's illustrations of Pamela are only one branch of development for the conversation piece. Highmore used the conversation piece as a means of representing the characters and incidents of a novel within a stage-like area.
Joseph Highmore, Pamela Writing, 1745
Source: Einberg, Elizabeth, and Judy Egerton. The Age of Hogarth: British Painters Born 1675-1709. London: The Tate Gallery, 1988.