What new information did you learn about Laertes in act IV that would help an actor or actress?
We learn in act 4 that Laertes is, underneath it all, a very passionate, emotional individual. This shows up in several ways. First, we find out how much he loved his father and Ophelia. Polonius might seem a pompous fool to us, but Laertes truly cared about him. It is when Claudius asks him how far he would go to avenge his father that Laertes says he would cut Hamlet's throat. Further, when Laertes finds out that Ophelia is dead, he cries, even though he says tears are womanish. He even goes away because his grief is beginning to overwhelm him. (Later, in act 5, he will jump into Ophelia's grave.)
Laertes's passions make him easy for Claudius to manipulate. Laertes is so eager for revenge on Hamlet that he goes along with the king's underhanded plots.
An actor playing Laertes could take heed of the emotional, passionate side of this character and emphasize that from the start.
In Act IV, Laertes comes home in a fluster because his father has been murdered. We learn that he is willing to cut a deal with Claudius to get at the murderer...Hamlet. He is resolved to act against Hamlet in a contrived plan to make it seem an accident. From this we know that Laertes is more of a person who acts than who thinks--the opposite of Hamlet. We also know he believes what he is told--not at all like Hamlet. Laertes seems a bit gullible in that regard. He also misreads Claudius since he does not think through how evil the King is, how far he will go, and what consequences would come of the mixed up swords.