First, it did not seem possible that he was old enough to die -- but then again, we all are, whether it seems possible to us or not.
Second, people kept referring to him as "Cheers star Roger Rees" or "West Wing star Roger Rees." While his performances in those two hit shows were memorable, for me Rees will always be Nicholas Nickleby, the lead in the epic early-Eighties production that played in London and on Broadway. He was ten years too old for it, at least, but so good that he made that difficulty evaporate.
He was a great talent and he will be missed.
Amazingly, all of the brilliantly filmed episodes of the eight-hours-plus Nicholas Nickleby Old Vic production appear to be on YouTube. I'm watching them now, and urge you to do the same before someone snatches them away, as Rees was snatched away. Life is a snatching-away, I guess.