The third quality you should look for in a prospective ghostwriter is the ability to mimic your writing style. Well known authors—particularly novelists—have distinctive writing styles, sometimes to the point that you can recognize their work even without seeing the name on the spine of the book. A ghostwriter has to be able to master many different styles of writing, to subsume his or her own “voice” into yours. It’s a bit like developing a good accent when learning a foreign language. A ghostwriter is not a “star” with a recognizable face, but more like the character actor you’ve seen in a dozen movies, never realizing all those parts were played by the same person.
Unless your prospective ghost already knows you well or you’ve worked together before, he probably won’t be able to write in your “voice” without hearing you talk and reading your writing. An experienced ghostwriter should be able to show you samples of work done in a range of different styles.
Note that in some cases the writer may need permission from clients to share this work, because either the work itself or the relationship with the client might be confidential. In such cases, be sure to ask for references who can attest to this ability.
If you’re interested in hiring someone who hasn’t worked as a ghost before, try asking her to rewrite a paragraph or two in the style of a famous writer: Shakespeare, or Stephen King, or Barbara Cartland, or anyone whose work is readily available and who has a distinctive style.