Dialogue is one of the most important things you’ll need to watch for on set. You might even be one of the only ones who has a script in front of them, has read the scene, and knows the plot. It’s up to you to make sure everything the writer wrote is accounted for, and make notes of all edits, alts, and adlibs to the dialogue.
When you get to set on day one, ask the director how important verbatim dialogue is to them. See if any lines are fact based, as those are usually a must for verbatim reads. If the scene or commercial spot is more comedy themed, or the talent is a comedian, then chances of adlibs instead of scripted dialogue is high.
When the take is rolling, follow alone with the dialogue while also paraphrase typing any alts. I find it easier to do this if I’m mouthing the dialogue with the talent. Remember, it’s your most important job to watch for dialogue. Most of the time I’ll get comments about how helpful it is to be saying the dialogue along with the cast, (instead of being laughed at for how silly I look mouthing the words).
If there is a tele-prompt operator on set, make sure to introduce yourself to them, and ask to have them sit near you as well. Any and all changes to dialogue need to be done ASAP, so having good communication with a tele-prompt operator is key for flow on set.
After the director calls cut, make sure to tell them about any dialogue issues. If it’s minor word alts, such as “okay” vs “yeah”, I might not mention those. But if there are plot points missing, I absolutely mention these dialogue issues as soon as possible. If I’m not sure, I wait for a good time to tell the director before they call action on the next take. It’s all about finding the right way to communicate with the director so that you can give your notes, be heard, and the talent can make the corrections without it coming last minute.
Remember, don’t give the actors any line notes/corrections unless you get permission from the director first.
Something I try to avoid, unless we are going very fast, is to give talent a note right before action. Give an actor a few seconds (preferably minutes) to get the line right in their mind before another take happens.