Here's an extension of the technique you propose, that may do more to
preserve the illusion of free will. Say you have a multi-path
plotline, with 4 possible actions the user could take to move the
plotline along. Put these in 4 geographically distinct areas which
roughly surround the viewer. Then the user can wander in just about
any direction, and fairly soon will trigger the forward motion of a
plot device.
Note that the only difference between this approach and traditional
adventure game puzzles, is that the "solution" is just to show up
somewhere.
More shells
landing on deck may be an interesting hint to the viewer that it's time to
head below. Once the viewer is in position, then the significant lines
could be delivered. Of course once this begins to happen, the viewer would
have to be prevented from leaving the area.
King's Quest frequently uses the technique of running a short movie to
move the plot along, that the viewer must receive passively. The
advantage is that the movies are usually really cool and great pieces
of work in their own right. The disadvantage is that the illusion of
free will is totally shattered.
I, personally, would like to get up and leave during someone's
long-winded speech, and go find something more interesting happening
somewhere else. Like what's happening on the enemy ship at exactly
that moment. As those fun folks at Apple say, "The User is in Charge."
Cheers,
Brandon