What's the subject of 20 questions in those issues where Jillian Grace is the centerfold model?
CREATE TABLE "table1_1566852_6" (
  "date" text,
  "cover_model" text,
  "centerfold_model" text,
  "interview_subject" text,
  "20_questions" text,
  "pictorials" text
);
SELECT "20_questions" FROM "table1_1566852_6" WHERE "centerfold_model"='Jillian Grace';