Some of it's a question of mind-set – if you're an experienced InDesign user, then you've already grasped the concepts of styles and page templates. My minimal experience (based on CS5 and FM 10) is that FM offers more for most documents unless you need really fancy lay-out … so the tricky bit (as usual) is working out where product A hides its equivalent for a command in product B; and the fun bit is discovering an idea in product A you'd never even thought of in years of getting to know product B.
The writing skills you bring to the interview are probably more important than the tools skills, imho. As long as you understand how to arrange different bits/types of information and how to make the presentation support the relationship between them, it shouldn't take long for you to add the basics of FM to your toolset. Learning the finer points takes practice, of course, but this forum is a good place for hints and tips.