First, to clarify what type of improv I study. It is ensemble improv of
the Chicago style, which was originally pioneered by Second City, but now
can be found in most major US cities (e.g. in New York, Chicago City
Limits is quite good). However, for a very long time now Second City has
not actually been improvising on stage. Rather, they use improvisations
in rehearsals as a way of developing scripts which they will then
perform. So your intuition that their performances do not seem
improvised is correct.
There is some tension in the Chicago improv community about this, since
most of the other groups do "pure" improv, meaning that nothing is
developed in advance. I can attest that this is indeed the case: I never
saw any group repeat any element of a performance. These actors are also
critical of Second City for "abandoning" improvised performance.
Second: I wanted to clarify this to help show why I think the adult
improvisation is an intrinsic "flow" activity, done for the process of
it, and I don't think the difference with children's play would be found
in "motivation" external to the performance. There is a consensus among
these actors that removing your mind from the immediate moment (for
example, to think ahead about what a certain character will do next, or
how a conflict might get resolved) is detrimental to an effective
performance.
Having said that: Much of the coherence of an improvised performance
comes from the relative stability of performed roles and personality
traits, goals, motivations of that character. And certainly the actors
reserve at least a corner of their mind to think about the broader flow
of the dramatic performance. Children at play aren't really concerned
with these "coherence" or "dramatic" factors.
Thank you for bringing up these issues!
Keith Sawyer
Department of Psychology
University of California, Santa Cruz
Santa Cruz, CA 95064