That interview has already been done. Bret Baier, is the most watched news person in the USA. Fox is number one and he is their prime time news person. Harris avoided him and FOX, until she needed a Hail Mary pass. He has a huge audience and she needs votes. She has no choice at this point in time. If she was ahead she would pass. But she needs a miracle.
What we will see has been pre-recorded. According to Bret Baier, the DNC wanted the interview done in a different time slot, than Bret Baier has. To interview Harris, he could not do a live interview, on his time slot. The interview was done and recorded at the requested time, and will be replayed during Bret Baier's normal time slot; 6-7PM.
Bret is a smart, old fashion journalist, who will be fair, respectful and hard hitting but makes his interviewee feel comfortable. He is not a hostile interviewer like in the DNC, but he is like a friendly pit bull with questions. This is why he is the most watch news personality. He does bot tell you what you want to hear and will even support the other side if warranted. The Left would love to have him, but would make him a mean biased pitfall.
The interview was done in advance which adds a wild card. The DNC may have offered Bret a sweet deal to come to CNN, since the interview will be edited down to 20-30 minutes to fit half his normal time slot, with normal commercial breaks. The DNC and spontaneity are antonyms.