Houses are about areas of life, not really a type of expression. Though that “area” can sometimes seem like a “way of expression” maybe. But to delineate a 12th house mercury, first it’s the sign and the conditions that go along with that, and yes, those darn aspects hehe.
But then we look at “how can this planet be used in this house?”. So we look at what kinds of things that house rules. The 12th holds places of isolation, or being “apart from” — think monasteries, hospitals, retreats, prisons, even things like working undercover in organizations dealing with those hidden enemies the 12th represents (think CIA ). So think about how mercury could be used in those areas. All the many ways that one’s mode of communication could be used there. What if mercury rules the 9th house of things foreign, but is in the 12th. Combine that with undercover or secretive work. Foreign places or languages. What if mercury rules the 5th of children? Maybe that mercury works with children with speech difficulties or disabilities in a residential place for that work. Don’t get attached to those specific examples, they’re just to show how to think about it.