For American media audiences, the Bill O'Reilly saga had pretty much everything, from sex and celebrity to money and politics.

For law firms and their clients, it was also a reminder: In the biggest scandals, there's nothing internal about internal investigations.

Companies facing major controversies have long hired heavyweight law firms, in part to signal shareholders that they're taking a crisis seriously. But when Fox News turned to Paul, Weiss, Rifkind, Wharton & Garrison to probe claims that O'Reilly had sexually harassed women at the network, that move alone made mainstream headlines—especially because the firm had investigated similar harassment claims against Fox News chairman Roger Ailes last year.