This are of course two common questions you need to be prepared to answer.
Why are you leaving
- I fully agree with Vlad, never criticize your previous employer, otherwise you somehow criticize your own experience and devaluate your profile
- Communicate positive arguments that gives you the will to look for next move. It's always better to meet someone drived by willingness to achieve certain targets rather someone leaving mainly because unsatisfied
Why this company
To be clear you should be able to re-use 80% of your arguments across different strategy consulting firm. (challenging environment, learning curve, CxO exposure, diversity of topics, etc.)
For the remaining 20%, try to collect / build specific arguments. Sometimes it's easier to communicate based on info you can have from local office business rather than global firm (size of the office, industry in particular where the office is strong, you simply met consultants, atmosphere at work, etc.)