I recently flunked the interviewing process for a Mobile Software Engineer role at American Express and thought it would be useful for future candidates coming down the pipeline to know what they are likely to be in for.
In terms of their set-up and infrastructure, AmEx has two distributed mobile teams named Itchy & Scratchy (each headed by a boss who you'll likely talk with for your initial phone screen & introduction). Most of the developers live near AmEx offices (e.g. Arizona, San Francisco Bay Area), but the majority of the 7 people I spoke with were definitely at home offices or at least working from home the day they spoke with me, so AmEx does have an open mind when it comes to truly distributed, remote work. Each of the teams is a mix of iOS & Android developers, product managers, QA folks, etc.
The mobile teams appear to fully invested in using all the latest Atlassian tools (BitBucket, Jira, HipChat, etc.). Daily builds go out for QA each day and a separate team builds the actual store release, which typically happens every six weeks (but it sounds like they are trying to get to four-week-cycle per release). While they are doing one week sprints, it's not a super formal Agile set up, but more of a Kanban-style process (that's how it was described to me; the way I interpret this is that there aren't super fixed, hard deadlines for getting tasks done). Because AmEx is a financial company handling customers' sensitive & personal financial data, a team member simply cannot use their own personal set-up for their engineering work but instead they'd be provided with an AmEx computer (where you do have admin access where you can install and work with various developer tools & utilities, but the IT department still has ultimate control over your company-issued machine). On the flip side, iOS developers aren't able to use Interface Builder to build their interfaces, but instead you'll need to get accustomed to some open source (found on GitHub) called PureLayout. You'll be writing your user interfaces entirely programmatically in code.
In terms of the process: after the initial ("culture"?) interview with a team director, the next round was three consecutive hour long interviews with six people in total (two people per round). It looks like the way they set it up was each of these technical interviews had one mobile developer in my specialty (who was doing most of the questioning), plus another technical-type person (project / product manager, back-end engineer) listening in and periodically asking questions. All of my interviews were done using a WebEx style tool named Zoom (there are mobile apps for Android & iOS, as well as browser plug-ins for desktop machines -- you'll likely need to use the desktop machines because you'll be doing Collabedit to show off your coding chops). It looked like a healthy range of different ages, maturity & experience levels (the oldest guy in the interview collection was probably in his late 30's).
I didn't think the questions were too out of line for my specialty, and I finished up the day feeling really good about the discussions & interactions (i.e. the "positive experience" checkbox on this Glassdoor review). But sadly it was not meant to be: I received the generic "good luck finding another job somewhere else" rejection e-mail within 24 hours of finishing the three hour long interview day. Since there's traditionally no feedback from these kind of big company interviews, I can only make guesses as to where I slipped up. Were I to ever do this interviewing sequence again, I'd recommend reading up on XCTest or whatever Unit Test setup your mobile architecture uses. I'd also be likely to not be as honest about my enthusiasm for working with a team of smart co-workers (I probably said something like "I'm excited about working with AmEx because I'm burned out working short term gigs as a contractor at other companies" -- and "burned out" might have been a phrase that turned off the two interviewers I spoke it to).
This was a job I really wanted, as it looked like it had the right kind of professionalism, responsibility and flexibility for the next phase of my career and I'm truly sorry I blew it. Perhaps you'll have better luck? Hopefully my experience flunking the American Express interview will help you to prepare to pass your interviewing day. If you find any of the information in my interview review helpful, please let me know by voting "Yes" on the "Helpful?" question below (this helps to motivate me to be as detailed as possible).