The King's Muscle
Thomas Cromwell (Leo McKern) is the brains of Henry's operation—a totally ruthless, unscrupulous, political operator. He starts out as Wolsey's secretary and ascends from there, reaching dizzying heights.
After Thomas More falls out of favor with Henry, Cromwell takes over as Chancellor. He ruthlessly persecutes More, bringing him up on charges of high treason when he can't bully him into signing the oath approving of Henry's marriage.
A Man for All Seasons portrays Cromwell as a slimy, backroom operator. Since More's silence isn't quite compelling enough of a reason to convict him, Cromwell gets Richard Rich to accuse More of bribery. When that doesn't fly, and the Duke of Norfolk shows that More dispensed with the bribe (which came in the form of a silver cup) once he realized what it was, Cromwell (apparently) gets Rich to perjure himself by falsely claiming that More had denied Henry's right to remarry.
Even King Henry VIII recognizes that Cromwell is a bad guy, and the movie's main villain (along with Rich):
HENRY: Those like Norfolk follow me because I wear the crown. Or those like Master Cromwell follow because they're jackals with sharp teeth... and I'm their tiger. A mass follows me... because it follows anything that moves. And then there's you [More].
Another Side to the Story
So, Cromwell's a jackal in A Man for All Seasons. But, in reality… well, it was complicated.
Hilary Mantel's prize-winning book Wolf Hall gives us a more sympathetic Cromwell: still kind of a bully, but someone honestly concerned with preventing England from falling into civil war… which is what would've happened if Henry didn't have a male heir.
Also, rather than being just a power-hungry cynic, Cromwell was a Protestant who had honest ideological and theological disagreements with More. (And since More executed Protestants for heresy when he was Chancellor, we can notice a certain symmetry between himself and Cromwell, right?) So the historical Cromwell probably wasn't as bad as he's depicted.
But, yeah, the movie's version of Cromwell is clearly villainous. Cromwell tries to pin More down, but doesn't understand that a true "Man of Principle" can't really be managed too easily. He says:
CROMWELL: And with a little pressure... with a little pressure he can be got to say so. That's all we need: a brief declaration of his loyalty to the present administration.
Incidentally, Cromwell would later be executed on false charges of treason, just as More was. What goes around comes around, eh?