It’s not enough just to flag a line as difficult.
It’s important to do so: “Tenor 2’s, watch out for that leap in measure 47!”
More important is to take the time to explain the context as to why the line is difficult.
It’s difficult because of a dissonance with another part. It’s difficult because it’s shifting tonalities. It’s difficult because the rhythm is intended to disrupt your expectations. It’s difficult because the interval is challenging for singers. It’s difficult because of the tempo, the tessitura, the pronunciation.
Whatever it is, take the time to explain it. The context makes it easier for singers to overcome the difficulty and arrive at accuracy. Too often, in our hurry to get to performance-ready, we skip the context and the reasoning. This is a short-term gain, maybe…but it’s definitely a long-term loss.