I saw a horror movie as a kid where a guy was killed by something (snake, maybe) when sitting on the toilet. It made me very anxious when using a toilet for a long time. I think the movie was dreamcatcher based on the Stephen king book but I was only 6-7 when I saw that part so I could be completely wrong.
You're remembering correctly. That's how the first character dies in Dreamcatcher. Stephen King was recovering from a car accident and was on oxy when he wrote the book if that helps explain anything.
"Car accident" is putting it mildly. He was walking down the road and a van slammed into him at full speed. He apparently went completely over the van and into a ditch. It's amazing he's still alive.