I’m a PIP assessor.
General advice: make sure you’re available for the call (I’m assuming it’s a call) at the right time, make sure you don’t have people making a racket in the background, deliveries coming etc.
Be prepared to spend beyond an hour on the call so charge your phone. Be prepared for the assessor to be running late, it’s very common that we are.
Don’t lie about anything; you’ll likely be caught out (some slip through, I’m sure, but we generally know when someone’s exaggerating or making something up).
Don’t try to shoehorn issues into irrelevant areas. For example, Activity 2 (taking nutrition), we don’t consider quality of diet, how many meals (more than one) that you’re eating in a day, what time you eat. It’s just wasting your time and the assessor's time as it'll all be disregarded.
Have recent medical evidence. Presumably you've sent all this in by now. It's not impossible to get an award with none, but it's unlikely. Very often, the DWP has already written to your GP and got a report from them which also helps expose any lies someone might tell.
As someone else said, this myth of "tell them about your bad days" isn't correct.
Long covid is a relatively new thing we're seeing, so give detail. Be specific about what you can and can't do. If you fail to give detail, your chances are lower, in my experience.