Jack Byrnes

Jack looks like a retired florist but was actually a CIA agent who interrogated double agents. When he meets Greg he gives him an improvised lie detector test and spends the weekend watching the young man. He distrusts Greg's nursing career and believes none of his explanations. He has hidden cameras in the house. In the end he accepts Greg as a son-in-law, and still checks the recorded footage. His CIA contacts find no record of a Greg Focker, because the young man's legal name is Gaylord. The secret meetings and the phone call in Thai are actually preparations for a surprise honeymoon for his daughter Debbie. His wife, Dina, reminds him that he disapproves of every man Pam brings home.

Jack drives the family to Miami in an RV and quickly clashes with the Fockers over their free-spirited ways. He is raising his grandson by the Ferber method, which Greg ignores. He suspects Greg has a secret son and orders a DNA test. At a dinner he gives Greg truth serum, and everyone turns on him. After being arrested alongside Greg and Bernie, he apologizes and tells Bernie about his CIA career. Jack invites Jorge, the 15-year-old son of the Fockers' former housekeeper and a close lookalike of Greg, to the engagement party, hoping Greg will admit paternity. When the truth comes out, he finds that everyone knew about Pam's pregnancy and kept it from him because they couldn't trust him.

With a heart condition now, Jack decides to choose a successor to lead the family and names Greg "The Godfocker." He sees Greg with the rep Andi at the new house and misreads it. They fight, and Jack has a heart attack. On the stretcher he admits he believes him and starts calling him "The Gregfocker." Months later, he and Dina decide to move close to their grandchildren. Jack also tries to talk Pam into thinking about divorce and reuniting with her ex-fiancé Kevin. Later he watches videos of Greg mocking him in a speech, and his distrust only grows.

Robert De Niro was cast on Universal's suggestion, after Analyze This. He came up with the lie detector scene himself.