For this depiction of Joseph spurning the advances of his Egyptian employer's wife, as recounted in the Book of Genesis, Guercino chose a three-quarter length format that presses the life-sized figures close to the spectator. He has filled the scene with the bed -- all rumpled sheets and opulent curtains. As the temptress reaches for the strong and handsome Joseph, he struggles vigorously to extricate himself. But she holds tight to the vivid blue cloak and sets Joseph spinning like a top out of his garment. In panic, he turns his imploring eyes heavenward, seeming to realize that even if he escapes with his virtue unscathed, he is helplessly ensnared in an evil plot; Potiphar's wife, bejeweled and confident, will later use the cloak to support her denunciation of Joseph as the aggressor.
With a delicate play of light on the seductress' profile, the artist shows the very moment of lust shading into treachery. If Guercino's narration is clear and eloquent, his presentation of the moral implications is more subtle: as this woman's beauty conceals her wickedness, so the visual lushness of Guercino's painting disguises a serious lesson about righteous conduct.
More information on this painting can be found in the Gallery publication Italian Paintings of the Seventeenth and Eighteenth Centuries, which is available as a free PDF https://www.nga.gov/content/dam/ngaweb/research/publications/pdfs/italian-paintings-17th-and-18th-centuries.pdf