If you want to preserve something, get it in writing, but if you want candor from someone, you'd better have a conversation. Even then, you might not get get candor but you'll stand a better chance. I recall a succession program that wanted executives to write down impressions of their subordinates. In this litigation-happy society, no one was willing to do that lest it be discovered.
There is another reason to opt for a conversation. It will permit you to hear the intonations and hesitation that often won't come through on paper. You can watch their facial expressions and measure their comfort with certain subjects. You can also pick up on minor topics that might be deemed as too trivial for a report but which will ultimately prove to be vital.
As much as many of us like reading, if the subject is sensitive, a conversation is best.