During a project, the PM will wear many different hats. Oftentimes stakeholders will attempt to advance a project's scope - sometimes inadvertently and sometimes on purpose. A good PM needs to be able to head off these changes where possible. This skill requires the ability to stand strong in the winds of adversity, because stakeholders don't always know what they really want or need. A good PM can help interpret their needs and deliver a product in the end that meets them.
When things go wrong, a PM takes some of the blame. After all, the PM is in charge of the project and should be making many of the guiding decisions - or at least helping managers come to decisions when necessary. Being able to clean up issues neatly and put out fires quickly is an absolute necessity. An effective PM can mitigate an unexpected problem, leaving little room for finger pointing, thereby keeping the project rolling along. Tact is required in many circumstances, since tempers often flare when surprises pop up.
Lastly, a PM needs to be able to sell solutions to difficult individuals. Sometimes a stakeholder gets an idea in their heads, and nothing can get them to change their mind. When a change needs to be made in order to help the project along, a PM has to have the ability to help smooth things along without ruffling feathers. This can be a difficult tightrope to walk. Strong personalities can make things especially difficult, and sometimes these are exactly the people that a PM has to "sell" a compromise to.