The latest official site Homing Beacon newsletter has some great new info on the character of Palpatine in ROTS. Here's a clip to encourage you to subscribe:

"He has a black irredeemable heart," McDiarmid says of Palpatine.
"There's nothing that can be said about him that's good. When we first
saw Vader in the original trilogy we thought he was the heart of
darkness, and nobody could be darker. But now we understand what
happened to him. And one of the exciting things about seeing this movie
is that you can follow Vader through Anakin's journey every step of the
way even if you don't agree with the choices he's making. In partm his
decisions stem from his traumatic childhood, and his impatient lust for
power. And it's that which my character takes and then uses again him."

"When you're playing a character of solid blackness, that in itself is
very interesting, in the sense that you have no other motivation other
than the accumulation of power," McDiarmid continues. "It's not so much
about not having a moral center, it's just that the only thing that
mattered is increasing power."

Yet even though Palpatine represents a truly evil character, McDiarmid
still attempted to seek out his human side, however faint it may
have been.