The shoulder angel often uses the image of a traditional angel, with wings, a robe, a halo, and sometimes a harp. The shoulder devil likewise usually looks like a traditional devil with reddish skin, horns, barbed tail, a pitchfork and (sometimes) cloven hooves. Often, both resemble the character who is in conflict, though sometimes they will resemble other characters in the story who are responsible or mischievous. The idea may have originated from the Christian concept of a personal guardian angel, who was often considered to be matched by a personal devil who countered the angel's efforts ( Especially in popular medieval dramas, like the 15th century The Castle of Perseverance. In both this and Christopher Marlowe's play The Tragical History of Doctor Faustus, of about 1592, the "Good Angel" and "Bad Angel" offer competing advice to the hero.
In several modern fictional stories, a character can be marked as especially evil or mischievous by receiving similarly bad advice from both shoulder figures, having a second shoulder devil instead of the angel, or being persuaded by the devil to kick the angel out.
There is a similar Islamic belief of Kiraman Katibin, two angels living on either shoulder of humans which record their good and bad deeds. However, these angels do not have influence over the choices one makes, and only record one's deeds. They are also called Qareen.
It's also possible to view this image in Freudian terms, with the Angel representing the super-ego (the source of self-censorship), counterbalanced by the Devil representing the id (the primal, instinctive desires of the individual).