The key to her choosing to speak through the piano is that no one is
expected to respond, at least not in a way that would make the exchange a
conversation or would imply that any real communication had taken place.
Such expression is *safe,* and for whatever reason -- it's not
important, really, what the resason is -- she feels the need to protect
herself.  That's why the Harvey Keitel character is such a compelling presence
for her:  He forces her to engage in a dialogue, to regard him in such a way
that she cannot refuse -- after all, the medium he chooses (physical
contact) isn't subject to the sheer power of her will, which, she says, is
so strong that it scares her.  (It's her will that causes her to put her
foot in the coil of rope attached to the piano at the last moment and to
plunge into the sea along with the thing. That IS a pretty strong will.)
Once a dialogue is established with Keitel, she no longer needs the piano and
her course toward a fuller *conversation* with the Keitel character is set.