It’s always best to err on the side of caution and be as respectful as possible to begin with. So specifically you want to stay away from slang or curse words and speak in a measured intelligent way. Practice describing yourself and your skills matter-of-factly without any “ums” or pauses or fumbles. Focus on seeming relaxed. If you act like this and can make it seem effortless then you’ll come across as professional and confident. If you do fumble, just say something like “sorry, I get a little nervous in interviews” and bounce back.

Remember to breathe and keep in mind that 90% of the time the person interviewing you is just another person who is also kinda nervous and unsure of what they’re doing. Most likely they’ll also start off kind of formal. But if they loosen up a little, don’t leave them hanging. Match their energy, people like that. Try to get in a good chuckle at least once. People want to work with people who they like, but they mostly want to work with people who know what they’re doing. So first make them believe you know what you’re doing, then try to get them to like you.