The two men corresponded for almost 50 years, in sickness and in health, sometimes quarrelling bitterly, till death did them part. We didn't get to hear every single word of their letters, just a selection. The letters were originally written in French and in modern English the translation was bound to include anachronisms, but it made their ideas and emotions immediately recognisable. They sounded equally intelligent. Voltaire in particular had a great sense of humour.
The only props consisted of two spotlit, modern office chairs centre stage which the actors came and sat on. They wore modern clothes and read the letters from ring binders. In fact I think it would have worked better as a radio play, although admittedly the actors' faces were interesting to watch as they read out the angry or the excited letters. Both men led troubled lives, but clung to their correspondence as a sort of consolation, it seems. The performance ended with "Frederick" reading out his eulogy to the philosopher who had died at the age of 83.