Introduction to Automated Dialogue Replacement.
If you’ve never heard (heh) of ADR, here’s an example:
Filmmaker IQ offers more context for the practice of adding dialogue in post-production:
Try It Yourself
Here are a bunch of techniques from top Hollywood dialogue editors, collected by Sarah Shachat from IndieWire, for getting ADR performances from actors which integrate seamlessly with their on-screen performances, and even their on-set audio.
Whether your dialogue is recorded during production or post-production, it will need post-processing – here are the need-to-know techniques: