When you need to chat with an employee individually, be sure to let them know what it is regarding. Even if it is only minimally.
Send them a message with a brief description of what you need to chat about.

   
No, "Hi"
No, "Got A Minute To Chat?"

That seemingly simple, yet nondescript query immediately sends your employee into a spiral of unknowns filled with worry and fear. People almost always go to the worst case scenario, especially when they think that their job may be on the line.

Think of your girlfriend or wife sending you this message: "We need to talk."

Those weird, vague pseudo-introductory messages are the workplace equivalent of that. The thing is, with romantic relationships, that statement is a death knell; whereas, with work, it is usually just an ill-thought-out attempt at saying something.

The huge takeaway is that if you can express yourself in a message, brilliant. Do so.
If you must discuss it via a video/telephone call, tell your employee what the chat is regarding prior to scheduling it.
It will keep your employee sane, happy, and productive.

Now, if you do need to fire an employee, you can still tell them something in a message prior to the chat so they know it is not going to be a good chat. For instance, "We need to discuss your recent performance issues."

Remember to put yourself in their shoes, and think about how you would feel when reading your own message.

Always write like you speak :)