Jack Butler, a 56-year-old bankruptcy lawyer who led the creditors’ committee, was instrumental in getting American Airlines’ parent AMR Corp. to the negotiating table with US Airways Group Inc. by prodding and coaxing Tom Horton, AMR’s chief executive, and his advisers. He also has pestered American’s unions and US Airways pilots and executives for over a year.

Companies under bankruptcy protection like to think they have wide latitude to control their reorganizations, especially because the law often prevents creditors from offering competing plans. But Mr. Butler’s relentless activism in the American case is a reminder that bondholders, employees and suppliers have the last legal word on what happens in a Chapter 11 case and can exert their will to influence the outcome.