This commercial for KFC on the year 2008 was trying to advertise their newly introduced Zinger Crunch Salad, and it was meant to show that it was going to be irresistibly delicious.
However, their way of advertising their product was not appropriate as it includes many people singing with their mouths full. This behaviour is inappropriate and is considered to be lablelled as bad manners in many countries, and this commercial was banned in the UK.
Albarracin and Kumkale (2003) recruited 162 participants and performed the task individually. They had them listened to tapes of persuasive conversation and a main factor is the direction of advocacy (whether or not the participants agree to the statement). It was found that the messages were less persuasive when participants listened to messages that they do not agree with.
In this KFC commercial, it shows socially undesirable manners, which most people would not agree with. So, in order to improve this aspect, the commercial could substitute the action 'talking with mouths full' with another action which indicates that their product is delicious (such as a Thumbs Up sign, which is universally known as a sign of approval).
Albarracín, D., & Kumkale, G. T. (2003). Affect as information in persuasion: A model of affect identification and discounting. Journal of Personality and Social Psychology, 84(3), 453–469.