I felt like preparing for the interview for domestic and general was pointless. The role is sold to you rather than you having to sell yourself for the role. As long as you are not a mute and am able to speak to somebody you will pass the interview.
Confidence and the ability to talk the talk are all you need
Research the company first, be confident, be a good listener, speak loud and clearly as you are interviewed in a group
Research about telesales
Prepare to prove you can work under fast paced working conditions
Read about the department to which you are applying in order to understand your specific role.
You shouldn't! Apply to AMEX but dress smart. If you do decide to down the D&G route just wear whatever, they will probably still entice you in with their "amazing incentive" structure.
Practice talking and answering questions in a busy environment where you can't properly hear yourself.