This is a no brainer! You're going to be developing apps for mobile so your smartphone is one of your main tools. I bet you're not going to have any problem answering this but it will be good that you demonstrate familiarity and knowledge on how to use more than one OS and brand.
Show interest on what they are doing, discuss some details and make some relevant suggestions about their projects.
One of the fastest ways to grow as a developer is to have other people read and comment on your code. Frequent code reviews mean the team is committed to getting better.
Security is always a very delicate subject especially when talking about mobile devices. Show your knowledge about security and expose your ideas about how to minimize security issues in the app they are creating. Get informed, was there a recent attack to a specific type of software? Mention it and be prepared to explain how you would have solved it.
It could take awhile before your mobile app is earning money (after all, have you seen all the free apps on the App store?). It'll take time for your app to be recognized and widely used by your target audience. Once that happens, you'll have the authority to charge a small fee for your app.
A mobile app developer should have some understanding of how to monetize a mobile app. There are several ways: in-app advertising, subscriptions, pay per download.
If your app will be free for the first little while, then your developer should have an understanding of mobile advertising networks.
If you are developing an app for an specific platform, you are expected to know which are the disadvantages of that platform. Here you can mention the technical issues you have found when developing for each platform, and also the ways to solve those problems. It's important that you're specific and provide examples.
If you're a large enterprise, you might feel more comfort knowing the mobile app developer you want to hire has similar experience.
The same goes for a startup business launching a mobile app. Businesses of different sizes operate very differently. The work atmosphere and culture could be very different from what the mobile app developer is used to, and therefore might not be the best fit.
If the developer has a keen understanding of processes, how paperwork and procedure is managed and budgets, then he or she may be a valuable asset to your team.
Most apps must be available on more than one OS, so experience reconfiguring or migrating an app from one platform to another is a very valuable treat. Tell about your experience in this field and detail the apps you have reconfigured and the solutions you have found to do it. If you don't have any experience, expose the reasons why you think you are technically prepared to do it.
Big players like Google and Apple are already using this approach to provide better UX while using their mobile platforms. Apple has Siri, while Google has Google Now. In order to enhance UX at some point, we can use voice for certain actions.
Cars can teach us a couple of basic things about designing with audio input for better user experiences. The first is that user experience design should not be limited to the usual graphic user interface (GUI).
For example, automotive apps could use voice to enhance user experience while the user is focusing on driving. Various car manufacturers have been integrating voice controls in their automotive infotainment systems for years.
Let's imagine you are building an app that will alert the driver when the vehicle is approaching a speed camera or a built-up area. All it will take for the driver to take notice and adjust their speed is a simple audio alarm. The car has no means to visually inform drivers that they are about to hit the curb, which is why audio warnings are used for lane departure solutions as well, and similar audio warning systems have been employed in aviation for decades.
Sound tends to be very useful when we go beyond the GUI, especially when it's necessary to alarm users and prompt them to act as soon as possible. This could be one of the examples how audio can enhance the user experience well beyond the screen.
More important than the actual development of an app is the testing itself. You don't want to launch an app that hasn't been thoroughly tested. Bugs and problems with features can happen, but should only happen during the testing period.
A mobile app developer should handle all of the testing and employ methodologies that tracks all features, establishes that they have all been implemented, and are in working order. Testing should be conducted during and after mobile app development.
It's important to seek outside help in mobile app testing. You and a mobile app developer will have been working on the same project for a while, therefore may be unable to identify flaws or drawbacks. Consider seeking an outsider's point of view.