I do not believe it would be a good way to practice because even with actors acting the way they are supposed (consistent body language and facial expressions) lets say conservatively 90% of the time, you are left with 10% wrong data. This 10% wouldn't be that bad except for the fact that it is actors trying to act correctly (meaning you would interpret what it looks like for a fabricated emotion to be a real emotion). This could be detrimental to many uses of being able to read body language such as telling when other people are lying.

My preferred method has been to watch court cases on YouTube where it has come out afterword whether the person was guilty or innocent. I watch these videos before i know what the truth is make a prediction and then read what the truth is. In this way I am able to get situations where the person is feeling real emotions and is likely to hide what there feeling with fake emotions.

After practicing like this for about a week i found that i could more easily discern whether people were telling the truth or lying, and it was easier to see what emotions they truly felt.

This may not extremely applicable to the real world because emotions felt in court rooms are particularly intense but i found that it allows me to get my mind to the point of being used to looking for emotion which has helped in the real world.

I should also note that i have read many books from Paul Ekman and have used some of his training programs.

If it is important to you how to learn to read faces I largely recommend SETT and METT where if its simply a curiosity you're unwilling to spend much money on i recommend checking out "emotions revealed" in your local library