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Getting an interview for an in house position is an accomplishment in and of itself. It's also an opportunity to impress, regardless of whether you think you're qualified. If you are qualified, or are a perfect match for the job on paper, you may just need to make it through the interview without embarrassing yourself.
One easy way to not feel embarrassed at an interview is to dress appropriately. The last thing you want is to show up dressed business casual when everyone you're interviewing with will be wearing a business suit. On the other hand, showing up for an interview in a plain, boring, suit when your interviews could be in shorts and a t-shirt could also be somewhat uncomfortable or embarrassing.
Corporate In House Interviews: Wear a Suit
Regardless of gender, wearing a nice business suit is absolutely the way to go if you're interviewing for an in house attorney position at a corporate employer. Even if the dress code around the office is business casual, you're an attorney applying for an attorney job and will likely be interviewed by another attorney or high ranking member of the company, and therefore you need to wear the attorney's uniform: a suit or pant-suit. The nicer the better, but don't go overboard. No tuxedos or fancy evening wear, and avoid bowties and obscenely fancy jewelry, unless that's your personality and you know you can pull it off (it takes confidence, not to mention nimble fingers -- for the bowtie).
The one caveat to this, for men, is the necktie. If you know it's a business casual environment, you can take the risk of going tie-less, if you have some fashion sense and can make it look good and professional. However, you can also play the necktie by ear. Before you go in, or while waiting for your interview to start, you can consider ditching the tie if you see the environment is business casual, or no other suited individuals are wearing them.
Startup In House Interviews: Wear Your Coolest Suit
Even if you are interviewing for an in house position with a young and hip start up where the founders wear shorts and t-shirts to the office, that doesn't mean you should leave your uniform at home for the interview. Rather, you need to up your fashion game.
Wearing a modern, slim fit suit/pant-suit with a stylish, fitted, patterned, and colorful, button down for men, or with the right pop(s) of color (or colorful patterns) for women, will likely help much more with your image than the typical off-the-Burlington-Coat-Factory-rack grey, black, or navy suit or pant-suit. Leave the top two buttons open (but only if it doesn't show too much skin); guys, skip the necktie for maybe a colorful pocket-square; ladies, forget gold or silver jewelry, accessorizing your tech wearables is likely to impress much more; and of course, for both men and women, remarkable/unique shoes will always make a good impression.
Startups tend to pride having a diverse and individualized workforce. Looking "hip" while wearing a suit can help you look like you belong, while also dressing to your potential role as an attorney.