No Plan Survives First Contact With the Enemy

... or friends, for that matter.

This saying, from the context of battles, applies equally to projects where outside influences are significant.  Initial planning swiftly becomes outdated, so it wise to plan at first in only broad brush strokes -- at a high level -- perform detailed planning as we go along.

For those who have limited experience as military command consider instead a game of chess (or checkers).  One might go in with certain intentions (play aggressively, use set plays when possible), but there is no way that you can plan out the whole game in advance!  You might plan a few moves ahead, but as soon as your opponent moves, everything shifts, and it's time to re-plan.

1972: Fischer challenges Spassky

As for friends (or family), suggest a plan and they'll want to get on board ;-) too!  Your plans will shift.  In a project this is the equivalent of an internal customer or product owner suggesting a better idea mid-way through the project -- and you acknowledge that it really is better.  Hmmm ... better re-plan.