In general, I would not worry too much about the questions at the end of the interview. They will (almost) never be the deciding factor between getting an offer or not - the interviewer will have already made up their mind.
That being said, some thoughts on what to ask:
1) Ask questions about the interviewer/their experiences at the firm/in consulting. People love to talk abou themselves (much more than questions about the company they work for), and will also give you a chance to connect on a more personal level).
2) Questions about specific practices/details that are not easily found online (e.g. if you are interested in the Financial Sectors practice, ask what the company/office in doing in this space.
DO NOT ask:
1) Generic questions that can be easily found online: this will make you seem lazy/uninterested in the company. Any question you consider asking (in advance), do some googling and only ask it if you can't find the answer easily
2)Questions asking to compare the firm to any competitors