Advice on interviewing tends to focus on the decision-makers: the interviews, supervisors, hiring managers, etc. who are going to decide which candidate to hire. But even in small companies, it's likely that the main decision-maker isn't going to be the person who coordinates the details with you — who schedules your phone interview or gives you directions to the office for an in-person interview.
I've helped coordinate more than one search committee as an office manager or administrative assistant, and it amazes me how two-faced some candidates can be. One candidate was all honeyed and obsequious in her phone interview, but when I'd tried to schedule that interview she'd been rude, demanding, and unhelpful, telling me I needed to find a new time for the entire search committee (12 people) to meet because she didn't want to do an interview on her day off. Do you think I kept that information to myself?
Good managers will ask their assistants for feedback on the candidates they've interacted with, but even when they don't there's a good chance that an assistant will mention a particularly rude — or accommodating — candidate. You're not going to land a job simply by being a decent human being to the person calling you, but you might just lose one by being unprofessional, whiny, demanding, sarcastic, or mean.
Besides making a good first impression, you may benefit in other ways from going out of your way to make an assistant's job easier. If a candidate is especially accommodating and pleasant to work with, I'm going to do my best to make things easy for them as well, whether it's getting them a preferred interview slot or bending over backwards to reschedule things if their flight is cancelled or delayed.
The candidate who calls and demands to speak directly to my supervisor to get an update on the process? I'm probably not going to mention that she's saying his name wrong.
Finally, keep in mind that this person may very well be a coworker of yours if you do happen to land the job. Making a good first impression, not just on the hiring manager but on everyone you interact with, will help pave the way to friendlier interactions and more help when you're starting out in your new job.