Really, you'll need to throw this ball in their court, since they are, in essence, doing you a favor. However, keep a few things in mind.
If you're interviewing over the phone, you'll want an absolutely quiet home or office. If you send an interview email, make sure you're online and available to answer any questions within 24 hours. Of course, in person interviews require that you be completely relaxed, on time and ready to interact. Think about the time of day when you're on top of your game, and schedule accordingly.