I thought I'd seen most of them, but it turns out I've only seen three or four per series so there are loads that I've never seen before.
Two things, for the first two series if the actor was someone that I recognised then that was definitely the killer. In season three somebody obviously caught on to that and the most well-known actor stopped being the killer. Second thing, Van Pelt's love interest is a guy name O'Loughlin, which is driving me insane because they pronounce it O'Loff-lan instead of O'Lock-lan. I did a quick Google and apparently that is how it's commonly pronounced in the US, but still it's really annoying every time they say it.