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 Studios’ Indeed Theater in New York City after a screening of their mafia comedy.

He said in their question-and-answer session, hosted by 69-year-old broadcaster Gayle King: “(Robert) whispers (on set), because it’s like, mafia guys they speak in whispers because it makes you come to them.

“And he says, ‘Listen. If you see anything I can do funnier, better, take me aside and just let me know.’

“(I remembered thinking), ‘Oh my God. What an amazing, generous thing to do.’”

And in the pair’s next take Billy took advantage of Robert’s request by poking fun at the star.

He added: “I go, ‘Come here. Is that all you got?’

“And then we just laughed and we just, throughout the whole movie you wouldn’t think that we would just laugh as much as we did.

“It’s hard to get through some of the scenes because they’re really funny, and he was extraordinary to work with.”

Robert’s mob boss suffers a panic attack in the movie and reluctantly seeks the help of psychiatrist Ben Sobel, played by Billy.

The film was a commercial hit and topped the box office on its opening weekend.

It also spawned the 2002 sequel ‘Analyze That’, in which Robert’s character is released from prison into the custody of Billy’s psychiatrist character.