I would talk to your director (is that person a teacher?) and see how they feel about the off book process. How long will you be off book before going into performances? Sometimes when working for a director who is the drama teacher, they may have high expectations, so be sure to find out that they expect before jumping to your own conclusions.
A good rule of thumb for lines:
If it's the very beginning and you have some time, and people are still learning, just note the big stuff. If they jump someone's line, if they reorder the words in their line so that it completely changes the context, or if they have a specific line that always gives them trouble. They are probably still learning, and know they are making mistakes. You'll be directing them to the areas that need the most attention, even though they know they need to review it all.
As you move along in the off book process, start to be more picky. Some people may argue with me that you need to note everything from the start, but, unless you have an army of people working for you, you'll be working on line notes all evening. Start broad, then hone in as the days progress. It's also extremely discouraging for someone to get pages and pages of line notes. This is even true for the most seasoned actors. Everyone makes mistakes in the beginning.
Be prepared to give line often when they call for it, and make a special mark in your script on those lines. I've noticed that some actors hit those same trouble spots time and time again, and if you have it notated that it's a usual line they call for, you'll be extra prepared to call it out when they ask.
There are also several methods of giving line notes. If you are interested to know my version, please PM me. I have very strong opinions on this topic.
Also, don't be afraid to correct the blocking. Be sure to talk to your director first, to confirm that the blocking you set in rehearsal is definitely what they want to keep, and that they are OK with you giving verbal corrections during rehearsal. Yes, it may disturb the actors and confuse them a bit, but if you know that someone needs to be somewhere and it affects other people, you need to make sure they do the right blocking. Your eyes are going to be everywhere, so it may help to review your script before rehearsal begins, just so you know what is coming up.
I hope this was somewhat helpful.
