First, take it easy. It's the first round, this means that you've passed the CV screeing and they're interested. The objective with the interview is to meet you and see who you are. They seek to profile you during the interview to see how well you fit with the corporate culture but also to assess your capabilities as a consultant.
Obviously, they want to know that you're motivated but also to know what motivates you (money, alone, is never a good reason. They will analyse how you reason, analyse and structure your arguments - there is no right or wrong answer.
As your position is not very senior, they are not expecting a rainmaker or a near-perfect consultant. I guess that they picked you because you have the right education, worked on the right projects and look like you have potential. No consulting firm will ever hire a consultant whom they do not think will advance, nor will they hire someone who lacks motivation, self esteem or humility - you're good, they know it and you have to show it without being arrogant.
Be yourself and honest. Putting on an act of what you think they expect will only end up bad. They will belive that you are the person described in your CV and that's who you should be.
It's only an interview