Can't really give you advice about the interview over and above the normal 'do your research, be yourself' type of stuff.
However, I guess the selling bit is all to do with their Skyshop; think about how would you get someone on a 45 minute flight to buy a sandwich and a can of coke off your trolley?
I like Jet2 - my flights from Gatwick to Manchester & back in November are less than £45 return all in; BA would have been about £100 or so. Though I can't see how they stay in business; the only time I've seen the planes more than a third full was when the strike at Heathrow was on & everyone was switching to Gatwick.
Good luck