Actress Nancy Balbirer, who lived with Jennifer Aniston in NYC, put on quite a performance the other night spilling Jen's secrets.At an event promoting the paperback release of a book called The Underminer: Or, the Best Friend Who Casually Destroys Your Life, Nancy -- and a host of others -- shared their real-life stories of their personal "underminers." Nancy's underminer was -- you guessed it -- Jennifer, who she met in the '80s when the two ladies were auditioning as extras on Saturday Night Live. At the time Nancy allowed Jen -- who she referred to as "Jane" throughout her performance -- to live in her apartment rent-free for a few months. Later, Jennifer stabbed her in the back. Here are some of the not-so-nice revelations about Jen:Jen, who worked as a waitress in a hamburger joint, was obsessed with her looks and would give her bikini waxes while lying upside-down on the couch.She advised Nancy to be "more f---able" on auditions and to stuff her bra with chicken cutlets from the Food Emporium. Before Jennifer would audition, she would always ice her nipples.When Jen moved to L.A., she got liposuction, a nose job and a hairline adjustment. Jen also slimmed down after going on NutriSlim.When Nancy was going to visit L.A., she called Jen to see if she could stay with her. Instead, Jen gave her numbers of local hotels, telling Nancy not to be so desperate, because it's unattractive, especially in a town like L.A.Hours after Nancy landed a recurring role on Friends, she was inexplicably fired but received her pay -- plus some. Later a producer friend told Nancy that Jennifer had her fired. The women never spoke again.