This is really tremendously sad news for me. It really hits home in a way almost no other actor's death could. For one thing, I consider him truly to be one of the absolutely best actors of my lifetime, maybe all time. He was absolutely tremendous in Lion in Winter (his tv version is doomed to exist in the shadow of O'Toole's movie, but to me Stewart and Glenn Close are an unmatched duet in that film), in I,Claudius, Logan and Hollow Crown (small role, but still). More importantly, as captain Picard he really had influence on me growing up. I started reading Shakespeare because of him. I bought a copy of a translation of Gilgamesh
because of this scene. I still consider Darmok one of the finest hours of tv in history.
And of course it was just a lot of fun to see him in many nerd-favorite movies, like Excalibur, Lifeforce or Dune (he's will always be my Gurney Halleck, even if he was only cast because Lynch thought he was someone else) and the more recent The Kid Who Would be King (ok, maybe it's only a me-favorite, but I really thought the movies was wonderful fun).
Some of his more curious cameos: he has a non-speaking role at the end of Smiley's People tv series as Karla, the soviet master-spy and Smiley's nemesis. And in Plague Dogs, one of the absolute best and most heartbreaking animated films of all time, he voices an unseen officer in charge of the hunt for the poor eponymous canines.