I would say some of both. The NSC-68 memo reveals a distinctly theoretical approach, in which a Soviet Union thought to be by nature expansionist must be stopped by the free world. On the other hand, decisions about where, when, and how to intervene were limited by ad hoc and practical considerations.
I would note that the USA got into trouble when it failed to maintain the practicality of its general approach–as when Kennedy said it would meet any foe, bear any burden, etc., and when Truman discarded Acheson’s policy of leaving South Korea outside the perimeter the US must defend.
.
