While they're acting, watch the screen you've attached to the camera and listen through headphones... You'll get a better idea of how "over the top" they are. Every take is a good take, but you'd like to see it a little different or want to try something just a little bit different, something didn't feel quite right for the character/story/edit/to go with that other take you did from the other angle. If dialog, you can spend more time on the singles and dial in their performances more tightly. This will also allow you to adjust the timing of the dialog more tightly to get a more natural flow as what you get in the master will be the actors searching for the next line and laughing at the process alot.
If you have shots where they're driving, you can read the line with the inflection you want off camera and have them parrot it back to you, have them listen to you, these will end up being reaction shots to the other side of dialog.
I have a cat toy that is a yellow stuffed star with eyes on a tall spring. I set this where I want the actor's eyeline to be so they have something to look at instead of the lens. Have them keep acting until you say "cut" or they will constantly look to you for approval the second after their last line is delivered. When shooting singles, have just the essential crew and the actor (singular) in the room so the stress level is lowered. Non-actors tend to let stress get to them more when the set is full of people than when it's just the necessary people to get the shots you need.
I treat it like I've heard pros talk about shooting love scenes, minimal staff means less inhibited acting. Stress is the killer with non-actors. Shoot fewer takes of the master, more of the coverage and allow your actors to pause and backup in the singles if they want to try a diffferent reading of a line.
Editing this is a chore, if you can get them to memorize the lines, great. If not, you'll have to piece meal them together, make sure you get footage of them delivering each line well in response to someone delivering the previous line off camera...and then have them wait til after the next line is delivered to cut and wait. This will give "acting handles" to your footage so you can J and L cut your footage to give the impression that the actor is carrying on a conversation with the other person in the room.
If you are shooting action, give them a destination, not just actions. If they have a goal, it'll force them to figure out how to achieve that and the "acting" will be more honest for the camera.
"Bring actor B the green pen from the table at the far side of the room" Will get them to walk to the far side of the room, approach the table, rifle through a drawer looking for the green pen (which may or may not be in there
) and reverse the process. Asking them to do all these steps individually will get them to "Act" walking to the far side of the room, "act" approaching the table...etc.