If someone tells you about a personal experience with a product, then you are likely to believe that person. If that 'someone' is a celebrity, you may be even more likely to buy a product that he or she is promoting. This kind of story about a personal experience is known as a 'testimonial'. Testimonials are used in advertising and political campaigns to appeal to the audience's sense of ethos, i.e. that which gives someone credit or a mandate. If a third, independent party has approved of a product or idea through experience, then the speaker's voice is validated. What's more, the audience may be able to identify with person giving the testimonial. This is why the person from the testimonial usually comes from the same demographic group as the target audience.

The 'Got Milk?' campaign is a good example of advertising that relies on testimonials and celebrities. This particular ad featuring Susan Sarandon comments on the importance of milk in providing the nutrients tha families need. She is speaking as both a film star and a mother. Many mothers can understand this importance to provide nutrients for children. Many people simply think: "If Susan Sarandon drinks milk, so should I!"