What sort of questions are common for a role such as this? What should I be prepared to answer? What would you want to hear from a junior iOS developer in an interview?
After that, the typical "Why do you want to work here?" type of questions will most likely pop up somewhere.
Also, I've learned that appearing as if you already have the job (to a specific extent) is a powerful way to stand out from the rest of the interviewees. For example, if the company already has an existing product, make a list of EVERYTHING that you think can be improved, and always provide a solution.
"This transition might appear to be a bit awkward for a new user, but if you had the subview pop up from the bottom here..."
If it's a smaller company, then make making a simple prototype might help. I would be a bit more cautious with this route, but it could prove to be beneficial in the right scenario.
Anyway, best of luck to you!
How would I prepare? I would look at the type of apps they create, or I might be working on and start trying to duplicate at least one of them. For instance, if they have a popular barcode reading app, create one yourself, maybe even add an extra feature. Show it to them. They'll see it and know two things a) you're capable of creating it b) worst case scenario you can add features to one of their products. aka, you'll never be completely worthless.