[vfulco2]

As others have said here, practice. In my career coaching service, I guide clients to practice 10-20x the length of the interview. Have a close friend test you on concepts or talk about the biggest potential areas for red flags on your resume or LinkedIn Profile as employers are looking to minimize risk. Interviewing is acting but playing yourself under stress conditions. So replicate the environment you will be in. Only 5-10 hours will make a world of difference.

To reduce stress, think about it as a conversation between colleagues (of different seniority). You are transferring information not in a court of law defending yourself.

Good luck.

[ergothus]

I teach coding on the side, and one of my interview tips for students is to practice saying "I don't know". Make it almost muscle memory so it doesnt throw you, because rare is the interview that wont try to find your boundary, and rare is the person that feels comfortable saying "I don't know" in an interview.

Sure, try to work in something you do know, or try to show interest in learning, but mostly just be comfortable. Confidence will be taken as skill, anxiety will drag down your display of strengths, and many companies are looking for someone that will be self driven enough to not need hand holding but will also be upfront about problems rather than committing then ultimately failing with little time left to correct.

I normally smile, nod, and ignore when someone tells me to practice a social skill out loud (that may be regrettably obvious in person), and I have long experience as a class clown so interviews arent very bothersome, but in this I say practice really does make a difference since the goal is not to establish what you can do but instead to make it boring and routine.

[wingerlang]

I recently had a few rounds of interviews, I think I said 'I don't know' a dozen of times. I did get an offer.