I've done a few internal interviews during my time at AXP. For the most part, they tend to be highly behavioral with what-if scenarios that focus on leadership and partner management as we are a very cross functional organization.
External interviews tend to have some skills based questions, case studies, but also include a heavy focus on personality and fit within the organization. The focus on fit is large, because people tend to move quite a bit. So a person that you hire, might move to another role down the road.
In general external interviews will include multiple rounds, with two to three interviewers each. Internal interviews usually include two to three individuals.
My advise for these interviews is to focus on whether or not you'd like to work with these individuals. Leaders tend to be the biggest factor for success at AXP.