When does it make sense to put someone on leave immediately after interviewing them?

It is always best to keep people at work when it is possible to do so without disruption or unmanageable risk.  With that said, when someone has been accused of extremely serious behavior, such as assault, or when the organization is sufficiently small that the complainant and respondent cannot be separated during the investigation (and the risk of continuing contact is great), a paid administrative leave is appropriate.

How do you deal with someone who repeatedly tells you that they don’t remember key facts?

I have a short fuse for people who don’t remember incidents that happened relatively recently and which, if they happened, would be memorable.  I find that saying that one does not remember is often a dodge.  I tell witnesses that I want to know what they DO remember about the time period or situation, and I encourage them to take a few minutes, to review their calendars or PDA’s to see if they can’t remember something.  I sometimes tell them directly that I find their lack of recollections suspect.  Of course, there are people who are clearly struggling to remember and cannot — and these folks, with authentic lack of recollection, are pretty easy to identify.