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.
Not hard at all, because even I was able to learn it, by myself. ;-)
But, joking aside, I can teach you the FrameMaker Basics in two days, and it may be even less if you are highly experienced with InDesign/Pagemaker.
Adobe has some Getting Started Tutorials too:
Adobe-Certified FrameMaker Instructor
Speaking as an almost complete FrameMaker n00b (I worked with PageMaker years ago), I've found it fairly easy, actually. It's a challenge switching mental frame-sets from, say, Word, but that's because Frame is built on a different set of assumptions. User control in Frame is much more granular than Word because it's assumed that you want that level of control--like the difference between driving a stick shift and driving an automatic. If you're experienced with InDesign, it should be fairly simple.
Comparing Word to FM is a bit like comparing a Rolls Royce to a small Ford. FM can do a lot more than Word and in a far more reliable manner. However it is easy enough to use the online resources (inlcuding these forums) to learn as you go. Yes FM can seem daunting, but unless you want to push the boundaries you can easily pick things up.