The most important thing to remember is that the composer is a senior partner. You cannot force a subject on a composer if it doesn't inspire him. He has to take the lead, you are an enabler, and you are creating the enabling conditions under which he can write great music. Your words are secondary. Many librettists in opera collaborations in the past have forgotten this, or not known it, or refuse to accept it and tried to get out in front of the creative process and it just doesn't work that way.