PROTEUS       Know, worthy prince, Sir Valentine my friend

This night intends to steal away your daughter.

Myself am one made privy to the plot.

I know you have determined to bestow her

On Thurio, whom your gentle daughter hates,

And should she thus be stol’n away from you,

It would be much vexation to your age.

Thus, for my duty’s sake, I rather chose

To cross my friend in his intended drift,

Than by concealing it heap on your head

A pack of sorrows which would press you down,

Being unprevented, to your timeless grave.                      (3.1.10-21)

It’s carefully calibrated, carefully rehearsed: know, worthy prince—deferential, a reminder of status, reputation that could be damaged—Sir Valentine my friend—not insulting Valentine, giving another reminder of the magnitude of what Proteus is doing here, apparently choosing duty to the Duke over his loyalty to his friend—this night intends to steal away your daughter. It’s imminent, this is a crisis, you don’t have long to act! And, he’s going to steal her away: he’s a thief in the night, come when you least expect it. (Silvia not mentioned by name, assumed to have little or no agency here.) Then another bald statement: myself am one made privy to the plot. I’m the only person who knows about this.

After this bare revelation of the apparent facts, Proteus spends more time, and more nuance, on the emotional and psychological aspects of the situation. First, I know you have determined to bestow her on Thurio—I’m an insider, I’m totally across your plans for your daughter, but, even more I know that your gentle daughter hates Thurio. Sorry, but that’s what I know. (This is sowing the seed of the idea that a match between Thurio and Silvia may be almost as undesirable as one between Silvia and Valentine.) But—setting aside Silvia and her feelings—and should she thus be stol’n away from you (stolen again, as a possession) it would be much vexation to your age. It’d be terrible for you! Really upsetting! (The assumption is partly that Silvia is the Duke’s heir; it’d be politically tricky. It mostly sounds like Proteus is saying, you’re getting on a bit, you don’t want to be dealing with this hassle do you?)

Then back to self-aggrandizement: thus, for my duty’s sake, I rather chose to cross my friend in his intended drift—because I owe more to you than to him (even though he’s my friend) I have made the really really hard decision to wreck Valentine’s plan and ensure that he doesn’t get what he wants. Because—and again, let’s be honest, you are in your dotage, more or less, and this could finish you off, couldn’t it, something like this, hasten your death? and I couldn’t live with myself if that happened, if by concealing it, I helped to heap on your head a pack of sorrows which would press you down, being unprevented, to your timeless grave. Proteus is being diabolically, nauseatingly clever here, representing the planned elopement as far more catastrophic that it would be, and the Duke’s investment in Silvia’s future husband as far more central to the Duke’s wellbeing than it ever could be. It’s much more interesting if the Duke isn’t played as elderly or doddering, and if there’s been no indication that a betrothal to Thurio is imminent. Proteus is an eloquent, plausible, psychologically astute chancer.