This advertisement from Time to Change uses source credibility to influence people’s attitudes towards mental health, and in particular, to encourage people to open up about mental health problems.
It features ordinary looking people of all ages discussing relatives, partners and friends of theirs who have suffered from mental health problems. The sources are thus portrayed as “just plain folks”; and such similarity altercasting will cause the audience to identify and empathise with the sources. As a result, the message will have a greater influence on the audience.
A classic study on similarity increasing source credibility is by Brock (1965) who found that confederates could persuade customers to change brand of paint more successfully when they mentioned that in a previous job, they had used the same amount of paint as the customer (similarity) as opposed to twenty cans (dissimilar).
Brock, T. C. (1965). Communicator-recipient similarity and decision change. Journal of Personality and Social Psychology, 1, 650-654.