Don't get stumped when an employer throws a curveball interview question at you.

Regardless of how qualified you are for a job, saying or doing the wrong thing in an interview can easily hurt your chances. Bestselling management author and CNBC contributor Suzy Welch says that to test you, employers "will try to get beyond the standard Q&A by using all sorts of 'techniques,'" in order to see if you're the perfect candidate. Welch tells CNBC Make It that there are three things you should make sure to do before, during and after an interview — as well as one mistake you should make absolutely sure to avoid:

1. Dress appropriately for the environment

While a suit and tie or high heels and a skirt may no longer be the traditional interview uniform, Welch says that doesn't mean you can totally relax your attire. She suggests asking someone you know about the dress code at the company or doing a simple Google search to find images of current employees. Then, she says, you should pick out an appropriate outfit for the occasion that's comfortable, but not too flashy. "At the end of the day, and interview, you want people talking about your ideas after you leave, not your outfit," says Welch. "Keep that front of mind, and you'll look just fine." Click here to learn more about how to determine the perfect interview outfit.

2. Be direct: Say you want the job

After you've discussed your background and accomplishments, Welch says you should end the meeting by telling the employer one thing: "I really want this job." Doing this, she says, won't make you sound desperate. It will show hiring managers your sincerity, courage and humility. "The person hearing it knows it's difficult to say," explains Welch. "They know it takes moxie, and they'll remember that." She says that in the end, you must remember that your goal in an interview is to "make your case" and "give it your all." Click here to learn more about why you should tell a potential employer how much you want the job.

3. Follow up promptly

Performing well during the interview is only half the battle. According to Welch, it's what you do after an interview that can "make or break your chances of getting an offer." In addition to writing a personalized thank-you note that expands on at least one point of discussion from the interview, Welch says you should edit your social media accounts and connect with your interviewer on LinkedIn. "After your interview, post intelligent tweets about your industry or the economy," she advises, "and please, avoid stupid Instagram pics." She also warns that when sending a LinkedIn request you don't want to "just click a button." Instead, she says, "say something about how much you enjoyed meeting and discussing x, y or z." Click here to learn more about how to follow up after an interview.

And one mistake to make sure you avoid