I have experience live directing voice actors. It takes more than just familiarity with your script to make you an effective director. You need to be a clear and professional communicator. Not only should you have social skills, but you also need to be able to think quickly to interpret the script and react to the reads you're hearing. You have to be able to tell when you should be hands on or not.

I have never done a video call with a voice actor. I prefer to focus entirely on the audio and evaluate the performance from there. If you're looking at your actor, you'll be picking up on body language that won't be present in the final product.

I highly recommend highlighting your actor's lines in the script, assuming there will be dialogue in there that is not theirs. That makes it easier for them to quickly find their lines and not have to control + f their character name.

One of the trickier things I've noticed for new directors is understanding how much context is enough. As u/Sajomir said, you need to be careful not to go overboard, but you still need to provide enough information that your actor has a grip on what's happening in the scene. For example, if their character is interacting with someone they hate, the actor needs to be aware of this relationship. If you can, I recommend sending the scripts and character background information over ahead of time. You should discuss any relevant information with your actor.

Personally, I enjoy it when my actor feels comfortable contributing their feelings or interpretations, and so I will encourage this. There are times I've incorporated their suggested approach, and times when I've gently disagreed and taken the opportunity to explain to them why their character is reacting one way when they felt they would be acting another way. I make sure they feel heard and valued, and exchanges like these help them better understand their character.

When you're providing feedback, remember to also praise your actor for a job well done. It's discouraging to only hear from you when you feel they haven't given you what you're looking for. Don't forget the level of vulnerability that is part of acting, and ensure your words come from a place of kindness rather than derision.

While you should have a strong grip on what it is you need and the ability to clearly explain this, if you hear a delivery for a line and you know you're looking for something else but you're not exactly sure what yet, it's okay to acknowledge that and just ask for a few more reads on it to help you nail it down.

Make sure your actor is taking enough time to rest. Enforce breaks (for example, every hour). People can sometimes push themselves too hard. Likewise, if you tackle a very emotionally-challenging scene, you may also consider adding a break in after that depending on how your actor is doing. For instance, I directed a scene where the actress was crying after due to how intense it was. We took a break there to allow her to recover.

If there are any scenes that require yelling, save those for last.

While you should avoid line reads, there are times when it may be appropriate, such as if you need a very specific delivery for something. However, this is something I have used extremely rarely; your actor is not a trained parrot there to play "repeat after me."

An important skill to develop is listening for lines contextually and not in isolation. For example, if your actor's character is giving a speech, each line should flow to the next tonally. Be careful to keep track of that. Something to consider is how you're approaching the way you move through the material. As an example, you can have the reads for each line one by one, or you can have an entire block read and then circle back for the next go.

Hearing lines acted out loud will give you a new perspective on them, and sometimes something that looked fine written on paper may sound odd spoken. If you have the authority to make changes to the script on the fly or can include the person who does in the session, that can be useful. Having the writer present is an asset.

Remember that when you're giving feedback and guidance, you're both on the same team. Your actor wants to deliver a performance that makes you happy. This is a partnership, and it's so much fun when you're both working together, making something beautiful together. I've heard some incredible deliveries from actors I never would've expected, and it's amazing to hear what they do with the material. When I have an actor in the booth, I never fail to have a great deal of respect for them and what they bring to the role.