For some people speech communicating is difficult. The thoughts are not always there when the words need to be spoken. Thank goodness for me there other communication skills I can use to grow my business. I prefer to go with my strengths, and speaking is definitely not one of my strengths.
I can write a good letter, one that expresses well what i need to get across, but it might take a few minutes to gather those thoughts. That works with writing .. you can mull on the topic for a bit, and rearrange the words on paper, until it flows.
That's not the case with speaking though. With speaking, there is no time to think about it., so unles you have your points on notes or cards that you can read, you might get twisted up, or worse yet draw a blank. I know too well about this.
Knowing the topic is a huge help

Knowing your topic and your audience, why they are there and what they are looking for, is not just helpful but necessary. Interview type speaking works better for me. I don't have to remember what I should be saying, I just have to answer some questions. Some communication skills are easier for me than others
I've also had to learn to listen better. Listening better helps in several ways. I can get a far better idea of what the person is needing to hear back, and I can also transport myself and my mind and thoughts into the conversation for a better outcome. It becomes more of a conversation for me, and not just me talking "at" someone.
You don't have to be an expert
It's not necessary to be an expert, but it does help if you have a general knowledge of whatever it is you are talking about.
Answering a request for information is far easier for me then getting up in front of someone to give a speech. It's another one of those communication skills that I prefer. Providing information and answering questions does not require me to know all the answers that second ...I only have to know where to get them. I can follow up with the person once I do that.
The followup is key. I know you've heard this before, and you'll hear it again. It's a huge part of the communicating process especially for a person like me that is better at providing information through various forms like writing and doing videos. So it's very important that I become a pro at follow-up. A professional follow-upper, and a professional inviter. By getting good at these things, I don't have to be an expert speaker.