John Everett Millais's painting depicts a scene from Boccaccio's Decameron. See the Walker Gallery Isabella webpages for full visual description and background to the painting.
Notice the ways in which the composition imitates the flat perspective of Early Italian art: the figures at the dinner seem to be piled upon one another, and the landscape viewed through the window does not recede into the background towards a vanishing point (which establishes three-point perspective in Later Italian and Renaissance painting.)
The models for a number of the figures were members of the PRB or their friends. Using a professional model would not only have been costly, but would not have been helpful to the contrasting aesthetic the Pre-Raphaelites wished to display. Professional models were trained in the poses and attitudes and style of the Grand Manner: they were not in line with the Pre-Raphaelite depiction of the body as realistic, grotesque, angled, and 'ugly'.