In Act II scene 1 of Hamlet, does Polonius really forget what he was going to say, or was he simply testing Reynaldo?
Interesting question. Of course, to answer it, as with any such question, you need to think of the kind of character Polonius is or how you would direct him if you were going to produce this classic tragedy. We need to remember that your job as a student of this play is (in a corny phrase that makes my students wince) to move Shakespeare from the page onto the stage as you study it. That means you need to come up with what you think Polonius is like as a character.
To my mind, you have two main options: Machiavellian advisor or doddery old man. There are certain stages in the play where Polonius seems to fit the former description, particularly when he advises Claudius about Hamlet and ruthlessly manipulates both Hamlet and his daughter to try and achieve his goal. However, at the same time, there are moments when it appears he is presented more as a comic figure, for example when he is trying to relate what has happened between his daughter and Hamlet to Gertrude and Claudius and they poke fun at his long answers and his inability to get to the point.
In this scene, note what Polonius says to Reynaldo:
And then sir does he this, he does: what was I about to say? By the mass I was about to say something: Where did I leave?
You either play him as being cold and calculating, deliberately testing Reynaldo as an underling in his paid employment to see if he is worthy of the job that Polonius has given him or as an old man who is slipping into his dotage and needs to be carefully reminded of what he was saying by those around him.