My interview with Gartner followed the format others have already outlined, so I won't bore you with the details - the process involved several telephone interviews with HR reps, people in the role, and hiring managers, followed by an in-person panel interview with the VP over the department, the hiring manager, and another manager. The panel interview required that I prepare and deliver a 20-minute presentation on myself, Gartner, the role, and why I should be chosen. I put a lot of work into it, and it was plain that the panel could tell I took it seriously.
Overall, this was the most intensive process I've gone through, and I brought a lot away from it. At every step, the interviewers provide you with feedback, and my impression was that the subsequent interviewer would compare notes to see how 'coachable' you are. They seem much more concerned with your personality and native talent than with your accomplishments or credentials, although those are still a factor.