Well, some interactive designs are mission-critical (airplane cockpit
controls, etc.), though most of us admittedly work on websites that may
fall apart but do not fall on people's heads.
Still, the point in common between architecture and interactive design
is that, unlike a conductor or filmmaker, both design an arrangement of
conditions that will affect, but not fully dictate, a set of future
events. How people behave in relation to what has been designed may
largely follow the signposts created, but users of both buildings and
interactive programs are unpredictable. Much as we may think we know the
most logical way for users to interact with a program, some people do
things that the designers couldn't have predicted (hence the utility of
usability testing). Just think how many architects thoughtfully provide
double doors at the entrance to their buildings, only to have the users
keep one of the doors locked! Or how many clean plaster walls get
covered with messy signs, phone lists, whatnot.