I went through the amex process earlier this year, targeting a senior individual contributor role on the consumer products side. The behavioral portion was a bigger deal than I expected, so writing this up.
Amex openly talks about their leadership competencies. Before your interview, look them up. They map their behavioral questions directly to these. The interviewers are not subtle about it.
Questions I got across the behavioral and values rounds: Tell me about a time you had to influence without authority to get a project over the line. Describe a situation where you disagreed with a direction your team was taking. What did you do? Give me an example of a time you had to make a decision with incomplete information. Tell me about a project you led that didn't go as planned. What happened and what did you learn? Walk me through a time you built a relationship with a difficult stakeholder.
All STAR-format. They literally told me to use STAR at the start. The hiring manager was taking notes and would prompt me when I skipped a part: "and what was the actual outcome?"
A few things that seemed to matter: Specificity over impressiveness. A moderately sized story told with precise detail landed better than a vague "I led a major initiative" claim. Ownership language. They noticed when I said "we" for everything. Some "I" is expected. What you'd do differently. The learning question isn't a trap. Say something real.
Culture-wise: amex has a reputation as a more corporate, process-oriented environment than a startup. The behavioral questions probe for people who can operate in that kind of structure without constant frustration. If you hate bureaucracy, be honest with yourself before applying.