One of the objections to project planning is time - it takes too long. On the other hand, it takes longer to fix a plan gone awry than it does to plan it right in the first place.
This phase forms the foundation for a plan that will hold up. First, identify who are the key players in your project. Look at two things: each person's importance to achieving the outcome of the project and the effectiveness of your relationship with the person. From there, you can construct a matrix and locate all the players on these two dimensions:

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Importance: How critical is this person to the decision being made and executed? Could decisions be made without this person?
Productivity: How productive are my interactions with this person? Do our interactions improve decision making or retard the process?
High-Leverage Relationship This person can contribute valuably to making the decision.
Decision Driver This person is critical to the decision process.
Once you’ve done that, get all the High Importance individuals together and make sure everyone sees the purpose and outcomes of the project the same way. When everyone is on the same page, you have an aligned core group.