Apollo turned to law firm Paul Weiss over and over again. The lucrative client stoked tensions at the law firm and opened it up to criticism about the close relationship, insiders say.

Private-equity giant Apollo Global Management has boosted the fortunes of law firm Paul Weiss to the tune of $100 million in annual billings and lifted its dealmaking profile, leading to nearly 100 mergers and acquisitions in which Paul Weiss advised Apollo and its affiliates, according to Dealogic.

But there has also been a cost to the cozy relationship, according to interviews with 13 people who have worked in its corporate department over the past decade.

Insider also interviewed several dozen others, including current and former Paul Weiss attorneys, consultants, and recruiters, as well as competitors, to offer an account of how Paul Weiss became so involved in Apollo’s business — a relationship one former firm lawyer likened to a plot line in the TV show “Mad Men,” when executives at a fictional ad agency scrambled to please its Big Tobacco client, Lucky Strike.