The walls come tumbling down this year for Hilton Lucas more ways than one. First, there's the so-called ""fourth wall,"" the theatrical term for the imaginary line that separates actors from the audience. Starting with tonight's second-season opener, Cosby crosses that line as Hilton addresses viewers, takes them into his confidence and even mugs at the camera over plot developments. And he's got a lot to react to, beginning with two new tenants. Daughter Erica moves back home; and neighbor Griffin loses his shirt in a record deal and takes up residence in the Lucas attic, which he begins to renovate. There goes another wall. After Hilton encourages Erica and Griffin to ""take risks and pursue their dreams,"" they do just that and the results become Hilton's nightmare. Erica quits her high-paying job at the law firm and moves home to save money and study to become a chef, and Griffin has to move into the Lucas attic after a business deal goes bad -- forcing him to rent out the lovely hous