I applaud your willingness to take on engineering management. Having made the move myself about 4 years ago, it's an often unappreciated form of contributing but I find it highly rewarding. It seems that most new engineering managers seem to get promoted from within since it allows them to leverage the respect they've earned inside the organization as an engineer. In no particular order, here's a few recommendations.
- Think back to the managers you've had and think about what they did well and what didn't work as well. The more you can talk about and have an opinion about what makes a good manager, the more you can show your desire and ability to become a good manager. Before becoming a manager, I spent a year going through my career and really looking in depth at my previous managers so that when I started I could try to use that to be better myself. What I found is that this made me very good at managing down and I was very popular with my team, but managing up was somewhat of a problem. So when you look back at your history and your previous managers, be sure to look at not only how your managers interacted with you and your teammates but also how your managers interacted with their managers and the rest of the org. This can be harder to see, since you're not a part of those interactions, but if you think back, you might remember at least some part of that.
- The most important part of being a manager, from my experience, is being able to deliver feedback. The more effortless and clear you are, the more easily you can provide frequent and minor course corrections as well as provide natural encouragement of desired behavior. It also makes firing/disciplining employees easy. For one, if you're giving constant feedback, those instances are much less frequent since employees can make those course corrections. But when it does become necessary, it's not a surprise. Either there was some major incident or there's been a long build-up where suggestions/warnings have been repeatedly ignored. When I've interviewed other managers, I've looked for their ability to deliver feedback and, crucially, their abilities to notice the things they should be giving feedback about. Many managers, especially new managers, just don't have the awareness to constantly be looking for small course corrections or the feel for when an employee needs a bit of emotional buoying that can come with positive feedback. Hopefully in your mentorship and lead dev experience, you've developed some of that awareness, so the more you can talk about that, the more you'll show you're ready. As far as delivering feedback, there's a lot of theory on the right way to do that, but it also requires practice. Read up on that and then find a friend who's willing to help and role play a few different feedback scenarios. You'll quickly get better with practice.
- I'm going to expand your question beyond the interview because I think it will help you with your interview. Because if you get hired, that's not the end of it. It's not a case of showing that you can do the work, getting hired and then just organically becoming good at it. Once you get hired, that's when you need to start diving into the theory behind the discipline of engineering management. If you can internalize that, then you'll be able to convey to your potential employer at the interview your willingness to work to become better. Try to show your interviewer that you have a plan for learning how to be a great manager and the concrete steps you'll take to achieve that goal. Because if you have zero experience and they know that going into the interview, that's the most they can expect from you.
- Not every engineer actually enjoys management. Many engineers really like knowing all the little details and have a hard time stepping away from that level of knowledge and only knowing the larger building blocks. If you can talk about your excitement to work at that higher level and willingness to give up that lower level, you'll at least convince them that you really want the job. Make sure that this is actually true, because it's hard to fake. But if you can show that enthusiasm, you'll subtly make a better impression.
- Lastly, try to stress areas of being a manager that you're already good at. For instance, as a lead developer, you've probably interviewed a lot of engineers. If you're great at hiring/recruiting, it makes being a manager a lot easier. If you can show that you're able to bring great engineers into their organization, that alone makes you a great hire. Another thing you've probably done is write 360 reviews for other engineers. If you can find one that you're particularly proud of, remove all identifying information from it, print it out and bring it to your interview as an example of the kind of thinking you'll bring to their organization.
Best of luck in the interview!