While I do not know the context in which this question was asked, I would say based on my own interview experience that it wouldn't be impossible to be asked something like this even if you have not applied to these other firm. The interviewer may want to test your ability to think on the spot about comparisons and issues you may not have considered before and also to test your broader knowledge of the legal market in London.
To answer it, firstly you definitely want to know what are the main firm 'types' - ie Magic Circle/SC/etc but also large/mid/small offices of US firms and also further differentiations between any big practice area/sector focuses - ie 'transactional' and 'disputes' firms. While you need not have a lot of specific knowledge about any of the firms within the broader groups, you need to know the main features associated with these categories. Thus, even if you do not know much about
Linklaters, you should be able to different it from GD merely by knowing what makes a firm 'Magic Circle': long history at the pinnacle of the legal market in England and deep ties with UK corporates; top of the market strengths across the spectrum of a full service offering; an expanded network of international offices; a structured training style; large trainee cohorts etc. The same points of differentiation will appear if the interviewer asks about
Clifford Chance,
Freshfields or
A&O Shearman.
Secondly, you need to know the unique selling points of GD and its practice area strengths, and you need to know what other firms those unique selling points/practice area strengths also apply to. If you have found a specific enough conjunction of why GD reasons, there should be no other firm they equally apply to and thus you should always have at least one point of differentiation - with Cleary, maybe the firms have in common strengths in cross-border disputes and mid-sized London offices; but Clearly does not have GD's market recognition for corporate M&A.
