Billy Crystal teased Robert De Niro about his acting abilities on the set of ‘Analyze This’. The comic, 76, and the ‘Raging Bull’ Oscar-winner, 80, co-starred in the 1999 mafia comedy, which saw Billy playing a therapist roped in to help Robert’s struggling mob boss character Paul Vitti with depression issues. Billy said he ended up teasing the Hollywood veteran after he took him aside and asked him to use his comedy expertise to give him advice on his performance if he thought there was a way he could be making it funnier. Billy made the admission during a talk alongside Robert at the Spring …