For Mrs. Clinton, becoming secretary of state would require her to sacrifice the independence that has come with a Senate seat and the 18 million votes she collected in the primary season. She has found it liberating the last eight years to speak for herself, not as someone’s spouse. But friends said she could still have her voice while subordinating her ambitions to Mr. Obama’s agenda.

“Hillary Clinton will always be seen as her own person,” said Mickey Kantor, a longtime friend who served as commerce secretary in her husband’s administration. “But you know, Hillary Clinton’s a terrific lawyer. She knows how to represent a client, and she’s good at it. And I don’t have any doubt in my mind that she’ll be a team player.”

Mrs. Clinton had to accept that she might never become president, a former aide said. “There’s a very small chance that she could run again,” he said. “You’re not going to be the president, so you want to make sure your next few years, which may be your last in public life, really make a mark.”

Two advisers to Mrs. Clinton said she was concerned about establishing her role in the administration before agreeing to the job. She wanted assurances that she would have direct access to Mr. Obama and not need to go through a national security adviser, they said. And she wanted the authority to pick her own staff at the State Department.

While Mrs. Clinton and Mr. Obama agree most of the time on foreign policy, during the campaign she made a point of highlighting their differences, seeking to paint him as unsophisticated. Now those differences will be brought into stark relief as she seeks to become into Mr. Obama’s emissary to the world.