Former FBI director James Comey prepared extensively for his discussions with Donald Trump, out of concern that the President was unlikely to respect the legal and ethical boundaries governing their respective roles, his associates say.

The associates recounted how worried Comey was about meeting Trump and recalled conversations in which they brainstormed how to handle moments in which the President asked for details of an investigation.

One associate referred to Comey's preparation as a kind of "murder board" - a phrase used to describe a committee of questioners who hurl tough questions at someone as practice for a difficult oral examination.

"He was pretty insistent that he would have to find a way to politically not answer it," one associate recalled.