If one were to use Marx Wartofsky's classifacation for tools:
Primary, secondary and tertiary, then improvisation could could
be either secondary or tertiary. For kids engaged in
improvisation (pretend), it could be tertiarly-suspending the
rules or to place oneself in an imaginary world. For a method
actory, improvisation could be a secondary tool, particularly if
one is using representations of life experience to discover and
control those experiences, later using them and the body as tools
to construct a truthful performance.
Wartofsky, M. W> (1979). Models: representation and the
scientific understanding. Dordrecht: D. Riedel
Bill Blanton