I'm not trying to pressure you, on the contrary, I haven't got a specific idea about what's doable and what isn't, so I'm posting everything that comes to mind to improve this scene.

What about:

Palpatine: You're sooner than I .... (Windu interrupts him, so you hear his voice only)

Windu: You're under arrest chancellor. The senate will decide your fate.

Palpatine: I am the Senate.

Windu: Not yet. I know who you are (here Palpatine starts to get up, it's a creepy moment that you mister, have achieved)... I know what you are.

Palpatine: It's treason, then.