During a confirmation hearing in 2015, Jeff Sessions asked Sally Yates if she can say 'no' to the president.

On Monday, acting Attorney General Sally Yates refused to defend President Trump’s immigration order due to its questionable legality. She was fired soon after.
Notably, expressing such reservations and acting accordingly was exactly what Jeff Sessions, the likely upcoming U.S. Attorney General, once pressed Yates to do, reports CNN.
During her 2015 confirmation hearing, Senator Sessions said, “You have to watch out because people will be asking you to do things you just need to say 'no' about. Do you think the attorney general has the responsibility to say no to the president if he asks for something that's improper?...If the views the president wants to execute are unlawful, should the attorney general or the deputy attorney general say 'no?'" 
Yates replied, “Senator, I believe the attorney general or the deputy attorney general has an obligation to follow the law and the Constitution and to give their independent legal advice to the president." 
The Trump White House apparently has a different view of the attorney general’s responsibilities. 
On Monday, it released an unsigned statement, noting, “(Yates) has betrayed the Department of Justice by refusing to enforce a legal order designed to protect the citizens of the United States."