The vast majority of screenplays I hear worked through the Lab have at least one thing in common. Many of them have narrative or descriptive passages that need to be cut down significantly. Screenplays are about economy of expression and making each word pull its’ own weight. Many descriptions of events or narrative passages about incidents may read cleverly, but they are not necessary or germane to moving the action along. That is like sand in the gears for a reader. You don’t want to bog the action down with excess verbiage.
Something we do in Lab and I strongly recommend is to read your script out loud. Get some friends together, cast it – and remember to get someone to act as the narrator. Often this seemingly thankless position falls to the weakest reader left in the group, after all the roles are cast. However when reading a full-length script, the narrator should be the strongest reader, hands-down. The narrator is the engine that keeps the reading chugging along, on-track and gets you home on time. Cast your narrator with the same care you cast your lead. Yes, it is that important.
Even if you aren’t finished your script but want to read say, the first act, then great. Cast the roles plus the narrator. Give as much direction to the actors as you wish. Give them sufficient time to read all the way through and answer the questions they may come up with. Then read it out loud. If the one voice you hear far and away more than anyone else’s is the narrator, then you know you have some editing to do. Believe me, the script will be better for it and future readers will be grateful for the edits.