Hello.
Firstly sorry, it’s a shit position to be in. I’ve been on the receiving end of it (no case to answer outcome) and I’ve subsequently trained to be an investigating officer.
If you have someone who can attend with you for support take them with you.
Difficult I know but try (at least during the interview) to keep the emotion out of it. Keep things factual.
I assume you have a copy of the allegations. Work through each one, where there is factual evidence provide copies. Go through your diary to check where you were, if you had meetings and to refresh your memory in case there are any potential witnesses.
During the interview answer the specific questions, don’t offer anymore, don’t “fill the silence”, just stick to the facts. Where there’s scenarios which are their word against yours don’t be tempted to “add anything”. If you don’t recall specific scenarios say that.
More widely look if you have evidence around their performance. Did you ask them to do something/tasks that they ignored, didn’t complete? Have they found it difficult to undertake reasonable management requests? For your own sanity can you think of an incident that might have triggered the breakdown in the relationship? Or can you think when things started to take a turn?
If it’s linked to performance for example have a timeline of when performance started to decline and how you were managing this - did this cause issues between you?
During the investigation meeting, try to keep calm. If you need a break ask, if you need them to clarify anything ask.
You can take notes in with you, which is why my starting point would be to work through each point raised and have your response.