Post by Timothy BrueningThe fake Moody says that many of Lord Voldemort's followers claimed that
they had been under the influence of the Imperiatis Curse when they did
his bidding. The problem was how to sort out the liars. This means that
Truth Serum (which Snape would threaten Harry with later that same movie)
must not yet have been invented! Otherwise, it would have been used to
sort out the liars!
Presumably Legilimency could also have been used to detect who was lying?
It may have been a question of scalability in both cases [...] And if
there are not many skilled Legilimens around (and they tend to have other
important jobs to do), then having one sit in on every low-level interview
of an alleged collaborator may not be a priority either.
Trust in the Legilimens may have also been an issue. Someone who has taken
Truth Serum can make a statement in front of an entire courtroom, leaving
little room for doubt about their truthfulness or for disagreement afterward
on what they said. But if you're using a Legilimens to "read someone's
mind," you're putting all of your trust not only in their skill but also in
their fidelity. If the Legilimens simply lied about what they found in the
suspect's mind, no one would have been the wiser.
Setting aside the question of whether an Occlumens could have sailed through
this kind of interrogation [1], the atmosphere of mistrust that followed
Voldemort's fall might have made it politically unviable for the Ministry to
assume that any particular Legilimens was trustworthy. (And getting multiple
Legilimens involved in minor cases starts to fall on the wrong side of the
cost-benefit tradeoff, like you say.)
[1] If someone had the mental and magical strength to use Occlumency in this
way--and if they were widely known to have such strength--it may have
strained credulity for them to claim to have been put under the Imperius
Curse. Of course, that in itself doesn't seem like it should be sufficient
to send someone to Azkaban.
Benjamin