You’re right about the ‘voluntary’ aspect, but I’ve made that point too in the text (# Proust and Freud):
“The remarkable aspect of Leone’s movies, is that a involuntary memory is made voluntary, by deliberately provoking it by means of an object.”
And I continue:
(...) the object has become a sort of Freudian fetish, only this time its function is not to provoke any sexual arousal, but to remind the user of the very moment that has become his raison d’être.” (...) “Harmonica can only preserve a minimum of coherence in the face of a childhood trauma through a form of personal madness, which translates in an identification with the harmonica: he plays when he's supposed to talk, and talks when he's supposed to play.”
In other words: Harmonica cannot live with the memory of the murder of his brother, but he cannot live without it either: revenge, killing the man who ruined his life, is all he can think of, it’s his raison d’être, his reason to be. It’s the harmonica (the sound, the feel of the instrument between his lips) that brings back the fatal moment, and it’s also the sound and feel of this harmonica that reminds him of his mission, therefore he deliberately uses it. In For a Few Dollars More the musical watche(es) have more or less the same function.
In relation to Proust this is new: in Proust’s novel, memories are involuntary. But that’s of course not the entire story, Proust’s associative style is very intricate. He started with the first part of the cycle and then immediately wrote the final part. All other parts (some 3000 pages) ‘grew’ organically by means of this associative style: one memory evoked another memory which evoked another memory which evoked etc. If you’d have a timeline of his novel, you’ll see the narrative jumping back and forth, from past to future back to present and again. Such a style would be far to complicated for a western movie, or a movie sec (but Leone came close in Once upon a Time in America; the style is also used, more or less, in recent films like Babel, Three Burials etc.). The only narrative style in world literature as intricate as Proust’s (in some aspects it goes even further), is Joyce’s stream of consciousness.