How do I generally hide an instrument that is not playing?
Are you talking about an entire movement, empty staves, or a cutaway score?
The first two are possible; the last, not really.
I wrote a piece for flute with a spoken word part, and I did it in Sibelius and have recently transferred an XML. What I want to happen is for the second line–the spoken word which I had on a single line “percussion” clef-- to appear but only when relevant. As I have it now, it kind of interjects to the flute part which I think is a mess. So basically I want to 1) have the second instrument but 2) hide it when not playing.
You mean a Frenched score, not a French score.
This is definitely possible in Dorico.
Layout options>Vertical spacing> Staff visibility
I took the liberty to modify the title accordingly, in order for this thread to be easily reached by people who need frenched scores.