Having a script will also help you manage expectations with participants as well as keeping the test on track. Ultimately you want the participant to speak more than you do, so the script will help limit what you say.
Usability testing is not complicated, but you need to make sure you cover your bases. There is a good chance participants in your usability test have never been part of one before, so you don't want to forget to tell them anything. Steve Krug shares a script in his books (as well as a free resource on his website! ). You can use this directly, or adapt it as needed.
In the script, you want to make sure you cover:
What you're working on/testing
How long you expect it should take
Remind the user that you are testing the product, not them (this is crucial!)
Ask them to think aloud (in other words, to speak what they're thinking)
Give them permission to be honest and ensure them that they won't hurt your feelings. Honesty is the most important.
Invite them to ask questions, but address the fact that you may not be able to answer all their questions during the test because you're trying to understand how the product works when someone is not there to tell them what to do.
Point out that you will be recording the session and confirm that you have their permission. If there are other people observing the session, be sure to let them know.
Ask participants to sign the consent form. (While they do this you can start the screencast recording)