By PR and marketing, do you perhaps mean publicity? I consider publicity to be one aspect of public relations, but I consider public relations to include many kinds of interactions and responsibilities, publicity being only one of them. Public relations includes everything from conflict resolution and community building to message control and customer (or client) service (and inquiries).
Marketing can also be looked at as the practice of building personal relationships, both within and outside your organization. You want to connect to your client, your industry, and the individuals connecting you to the end users.
I'd also say that marketing isn't the same thing as publicity, marketing itself is just another aspect of public relations, and publicity is included in your marketing strategy. Marketing can mean your entire strategy from designing your product to communicating it to your market and then delivering it to clients, and your marketing strategy can even include the consumer experience after the clients have the product in their possession and are using it. Publicity more specifically means mentions in outside press.
I think for all these things, excellent communication skills are a very high priority. While confidence is always important, it's also possible to come across as too strong or aggressive. Coming across as professional and dedicated is also important. That can mean triple checking your spelling and punctuation, or writing in the style appropriate to your audience. It shows clients and coworkers that you care, and it reflects better on the product or service you're representing.
Edit, I just noticed that you weren't asking for tips. I apologize, I misread your post the first time through.
