Completely agree with everything that Jacob just said! Some additional tips:
1. Expect a mix of competency and commercial questions. I think most of them are competency questions with one commercial one so prepare for some of the typical competencies that firms look for like teamwork, time management and resilience. Also prepare for the most common type of commercial question i.e. "Tell me about a recent news story and how it affects the firm".
2. Practice - Maybe by talking aloud to a friend on skype or if you have a premium membership on TCLA, there's a great VI simulator.
2. Make full use of the preparation time given to you. Most VI platforms give you 30 seconds to prepare your answer, so have a paper and pen next to you in case you need to jot your answers and key points down.
3. Be as succinct as possible. The tough thing about VIs is that it's really difficult to get your point across on why you want to join a firm you really like/how you showed teamwork in 1.5 minutes. So keep a simple structure, use clear pointers like "Firstly, secondly, thirdly" if need be.
5. Projection - However loudly you speak in real life, speak twice as loudly because I've always found from recording myself that audio can sometimes end up sounding muffled no matter how good of a recording device I have. Also speak slowly and clearly and try not to rush through your answers!
6. Look at the webcam when you speak and not yourself/the screen (can feel a bit awkward doing this at first, but do try practicing it). It can also help to put the webcam at eye-level using books/a stand like Jacob mentioned.
7. Enthusiasm - Smile a lot. Even if the VI is the last place you want to be (which for most people it is), make sure that you're desire to work at the firm is really conveyed in your body language!
I don't know anyone that doesn't dislike video interviews and I was an absolute nervous wreck doing mine. But even when I stumbled, I always tried to remind myself to just take a deep breath and if really need be re-start the sentence or what I was saying. Like Jacob said, I think GR know how tough VIs are on everybody so don't be too hard on yourself if you slip up slightly.
Good luck with your VI today, I'll have my fingers crossed for you!