I've made quite a few mistakes when hiring people over the past 20 years.
Most of them were in how I approached the interviewing process. I made too many assumptions, didn't follow through on asking for credentials or samples of work, and avoided asking the toughest questions when a red flag came up--just to make sure I was being nice. Here are a few lessons I learned after hiring people in companies large and small.
1. Keep asking questions until you're sure
One of my biggest regrets in hiring came a few years into my career running a large department. I was too cavalier about it, and over-confident.
I met with the candidate in the company cafeteria and felt pretty sure she had the right skills. In fact, I had made up my mind about her even before we sat down for a second interview. We mostly talked about salary. Yet, if I repeated the process again, I'd ask more questions. I would have likely discovered that this person had some personal issues that would cause productivity problems on the team.
Can you ask too many questions and overload a good candidate? Sure. Can your questions get too detailed? Yes. However, in my experience hiring dozens of people, the good candidates can withstand the scrutiny, and may even enjoy it.
2. Go ahead and make the credentialing process more rigorous
The steps a hiring manager goes through with a job applicant remind me of what it's like to go through a rigorous editing process. I am currently in the middle of one that is winning all the records for revisions. At times, you almost feel like you want to just bow out and say never mind. (The truth is that I enjoy this process.)
If your candidate really wants the job and can withstand some rigor, that same person will also persevere through tough work challenges after you offer the job. I'm not in favor of IQ tests (or really any other aptitude tests) because they only reveal so much about how that person will perform, and the candidate might just be really good at taking tests.
You can't exactly assign real duties, but you can certainly talk through a few work scenarios. And, depending on the job, you can ask the candidate to submit samples--e.g., for a video production job, you can ask them to make a video. Or for a communications job, ask the candidate to write up a press release (or two). Use credentialing as a way to see how the candidate will actually perform in the job.
3. Ask the hard questions and don't be afraid to pry
As I look back on the people I hired who turned out to be poor matches for the job, my biggest regret is that I avoided asking more questions when something came up in the interview process that was an obvious red flag.
The person who wants to be a designer makes a sarcastic comment about how they worked with a former employee, and I overlook it to avoid getting too personal. Or, I ask questions about a former boss and don't relay my personal style of management and get their opinions because I don't want to scare the candidate off.
Asking tough questions of a new candidate once you see a red flag is the best way to determine how that person will perform when that same situation arises. By pursuing a line of questions that could be uncomfortable, you'll avoid the even more uncomfortable process of having to fire that person down the road.