Giving a test lecture, what should I keep in mind?

Off the top of my head, things that are different about a lecture rather than a presentation:

• You're not simply relating stuff you did to your peers : You're teaching people who know less about the topic than you do (even if you only know what you got from the textbook when preparing!). So think carefully about the intended audience, and how to explain the concepts at the right level. This will take time. Fortunately...
• You have time. You have an hour or more rather than (say) 15 minutes. This means you need to convey less content per minute, and you can spend more time building understanding step by step. Try to think about helping your students to develop a mental model that they can fit the new concepts into, rather than just telling them facts.
• Because the students are having to assimilate completely new concepts, don't overcomplicate - keep the necessary cognitive load to a minimum by omitting unnecessary details. Don't lie to them, but it's OK to say, for example, "this is a simplification, but it's a useful way to think about it at this level. We'll come back to it in more detail later.".
• Because you have more time, you'll also need to think more about (and spend more time on) structure. Think about pacing - don't talk for an hour. Instead, especially if you have a relatively small class, try to get some discussion going, or set an exercise. This may not be applicable for a demo lecture during recruitment, though.

If you have a small group (<25 or so, as may happen for postgrad courses) then it's more like a lesson than a lecture and things can be far more interactive.

The more you know about your audience the better your presentation/lecture/class will be (not just on this occasion).

Will this be a regular class in a regular course, or a simulated class with some students and some volunteers or (I hope not) a group of professors pretending to be students?

Can you politely ask your prospective employer for information about your audience and the context? Is this a regular lecture? For what course, at what level? What have the students covered recently?

At the lecture, be sure to involve the students. Check that they understand the prerequisites for what you are covering. Ask if your notation matches what they are familiar with. This will take time. Don't worry about "covering all the material" - pay more attention to keeping the students engaged.

I have often thought and said, only half jokingly, that you should think of each hour's class or presentation as live theater. Your responsibility is to assure that your audience leaves thinking it was an hour well spent.

You sound like you have a good grasp on how to present information to a large group, so I expect that you've already begun to find a style that works for you. I won't address those elements, then, even though they're important.

A presentation at work often involves people who are already attached to the project you're working on, and a presentation at a breakout session of a conference often involves some self-selection in the audience. You can already assume some level of professional engagement with your topic (or personal interest in it) from the people you're addressing. Academic lectures are different, though. One might think that students who have paid tuition to attend class are already motivated toward the topic, but that's not the case at all.

Accordingly, the most significant recommendation I can make is to address yourself to student motivation. How does your presentation matter to them or to their interests? To their patterns of thinking? To their work? Why do they need to hear what you'll be saying?

In other words, Why are you not wasting their time?

There are a lot of different reasons why people fail to address student motivation, but they invariably all end in the same place: disengagement. Even entertaining or clever presentations that feature the latest pedagogical practices fall flat outside of the few students who are intrinsically motivated or manage to grasp the material's importance without it being explained. It's not just making application. You need to connect it with something they already care about.

I often find that it's helpful to start with a set of questions or a brief exercise that exposes a common misunderstanding about the topic. Even better, this introduction will demonstrate that our usual approaches/analyses are contradictory or otherwise problematic. Once it's established that--"Yes, you too!"--the students have something to gain (or lose) from the presentation, then they're motivated to engage with me for the rest of the presentation. The exact nature of your demonstration-to-prove-relevance depends a lot on your specific subject, but I encourage you to construct it in such a way that the students discover its relevance rather than being told its relevance.