He reportedly died from complications for an undescribed surgery. Here's the story from the New York Times:
https://www.nytimes.com/2017/02/26/arts/bill-paxton-dead.html?_r=0
My wife came to know him from Big Love, but I knew him from all kinds of genre stuff:
Terminator (some kind of gang leader)
Aliens ("Game over, dude!")
Marvel's Agents of S.H.I.E.L.D. (John Garrett)
... and some other stuff, like Apollo 13 and Twister. He seemed to be everywhere for a time, and I recognized him and enjoyed him in whatever role he was in.
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I remember him best from Apollo 13, one of my favorite movies, as it has people fighting against the odds to prevail, but there's no villain.
I remembered yesterday that he was also in Titanic and True Lies.
Unlike many actors, he happened to be in a lot of stuff I watched (or I watched a lot of stuff he happened to be in), so this R.I.P. is sadder than most for me. I've known the guy's name since Aliens, and never been disappointed when he appeared.