Most of the film was lit using candles rather than conventional movie lights. In order to keep the effect of candles but still get enough light, cinematographer Alexander Melman designed a special piece of equipment, a stand than held a bank of candles and a reflective backing. These were known on the set as Birthday Cakes.
Unlike most period films, this one was shot almost entirely with a hand-held camera. The two most notable shots with a fixed camera (not a hand held one) are the two panoramas of the interior of the theater, which was intentional.
Looks closely at the back of the person being prodded and whipped around 1:17:10 (in Dr. Bendo's room shortly after Alcock pees into a vial), you can faintly see quite a large tattoo in the small of the lady's back that hasn't completely been covered by the make-up.
The Earl of Rochester reads an insulting poem he wrote about Charles II which implies that the King is impotent, and insists this "is true." In fact, Charles II was a noted womanizer who fathered at least a dozen illegitimate children by seven mistresses.
In the playhouse, after Harris announces the kings arrival, Lord Rochester starts to remove his hat with his right hand on the crown of the hat. In the next shot, he is removing it with his left hand on the brim of the hat.
Rochester: Oh, written a new play has he? All those afternoons pretending to slope of and roger his mistress, like a decent chap, he was lurking in his rooms poking away at a play. That is disgusting, George.