How to prep for interviews at a startup?

At a high-level:

• Startup interviews have more practical coding (i.e. you need to build a small app or endpoint live)
• Big Tech interviews are more data structures and algorithms focused
• Startups will place a heavier signal on behavioral questions as passion for the product and culture fit are much more important at earlier company stages

Now let's talk preparation techniques:

• Ask them what the interview will have - This basic technique is way more important for startups as it's much harder to get this information through traditional means (network, Glassdoor, etc). So straight from the horse's mouth is often both the best and only way you have to predict what's going to be asked of you as David mentioned. For communication techniques on how to extract this information, check out this other discussion: "How to figure out what's going to be on an interview?"
• Build side projects - Startups care much more about your raw coding ability than Big Tech does for obvious reasons, which is why startups will often have a practical coding round where you need to build a small app/website/back-end endpoint live. I work on Android, and pretty much every interview I have done for a startup had me build a small Android app from scratch. By the time I interviewed for Robinhood (4 months before IPO), it was just barely a "startup" anymore. However, I still had to build 2 Android apps from scratch during my interview (1 during phone screen and 1 during onsite). And when I was the Android lead at Course Hero, I wrote the Android engineer hiring process which included a round where we made the candidate build a small Android app from scratch.
• Keep your IDE warm - Make it so that if your startup interview does have a practical coding round, you don't need to waste 15 precious minutes setting up your environment. This shouldn't be a problem if you're doing side projects. I actually failed a couple candidates back at Robinhood, because we told them that they were going to have build an Android app from scratch, they didn't prepare their Android Studio at all, and they ran out of time during the interview because they wasted 1/3 of the round upgrading their Java and downloading emulator packages. Don't be like them.
• Get to know the company - I mean, actually get to know the company: Don't just spend 5 minutes on their website and call it a day. Download their app and use it for 15-30 minutes, read whatever press they've generated across TechCrunch/Wired/Verge, look up the founders on LinkedIn and go through their stories - Make it so that when they inevitably ask you "Why are you interested in working for us?", you can have a non-embarrassing discussion that shows the effort you put into understanding them. There's a good chance you'll be talking with the Head of Engineering or even a cofounder if you're interviewing for an earlier stage startup: You definitely don't want to come off as a person who just wants a job in those conversations.
• Be careful with data structures and algorithms (DSA) - While Big Tech interviews are infested with DSA, startups aren't really (at least from my experience). Of course, many startups will ask DSA questions, but the hiring signal there is way weaker for startups as they actually need engineers who can run the ground running and push a ton of feature code. If the recruiter tells you that there's going to be DSA problems, by all means grind. And if they say there isn't going to be any, just don't do DSA at all. If they say neither and you have 0 idea if DSA will be on the interview, keep the LeetCode prep on the lighter side. I would stick to LeetCode Easy + Medium only and spend just 30-45 minutes per day.

Wrapping it all up, the following are much more important tactically for startup interview prep:

• Side projects - To learn how to get started with these, check this out: [Course] Build Side Projects With 500k+ Users: Coming Up With An Idea
• Behavioral questions - I made a course about this too: [Course] Master The Behavioral Interview As A Software Engineer

For getting the interview to begin with, check these out:

• "How to find and connect with scrappy startups?"
• Stop Getting Ghosted With Your Reachouts - How To Write A Great Cold Message
• "How to get interview calls?"

Have you tried looking up questions for that company on Glassdoor or Google? Every team and interviewer is different, especially when you move into the start-up world. That said, you might be able to find questions for the company on Glassdoor.

Overall, your best bet is to talk to the recruiter to clarify expectations around the interview process. They should want to help you prepare and give you clear expectations, especially if this is a place you would want to work. Most recruiters would also take this as a positive sign that you are interested and taking their interview process seriously.

Beyond that, general interview preparation (e.g. understanding algorithms and data structures) is always important. Also, especially for a start-up, look at the role you applied to and be ready for questions related to the requirements and responsibilities of the role.