A plan describes the day you hope for. Everyone who built it knows its weak spots, but while it still looks good, raising them sounds like doubting the plan or the person who wrote it.
A pre-mortem changes the question. Instead of asking what might go wrong, you state that it has gone wrong and ask what happened. Failure stops being a possibility to argue about and becomes a fact to explain, so naming a cause becomes the useful thing to do.
For a leader, the hardest voice to get past is often your own. Enthusiasm for a plan, yours or your team's, is what makes its gaps hard to see, and a pre-mortem gives the quieter view a turn before the plan is locked in.
Write down what comes out, ideally in a decision journal, so you can check later which failures you saw coming. Then change the plan for the causes that would hurt most.
The practical tell is a plan with no slack in it. If nothing in it allows for a late start, a missed handover or someone off sick, it was built for the perfect day.
Tom's excitement about meeting clients and filming them at the show was off the charts, and the schedule worried me. So I asked him to imagine the show had gone badly and tell me why, then gave him the first reason myself: our first client is busy, we run 30 minutes over, and every client after that is kept waiting. Playing it out showed us we'd planned for the perfect day, so we built a more flexible schedule and shared it with every client we were filming. On the day, clients ran late, just as we'd imagined, and the new schedule held, just.