Lisa Kudrow says life behind the cameras of the "Friends" set wasn't always as warm and fuzzy as it looked onscreen ... claiming the show's writers' room could be downright toxic -- and even included alleged sexual talk about her costars. The actress -- who played Phoebe Buffay for all 10 seasons of the hit sitcom -- opened up in a new interview with The Times of London ... revealing the off-camera environment could get "mean" and "brutal." "There was definitely mean stuff going on behind the