Whatever you do, do NOT read about Don't Look Now. If you choose to watch this, you must go in "site unseen", just like with Triangle.
The difference is this: Triangle is more "engineered" for what it is, even if it's engineered to perfection. This is why I feel Triangle is one of the greatest horror/mystery movies out there. What about Don't Look Now?
Don't Look Now is more of an old-school art piece that has more loose symbology and metaphorical meanings. In Triangle, everything can be put together with horizontal deduction, it seems like. With Don't Look Now, it's more ambiguous. For instance, sometimes a flock of doves just means a flock of doves. Oh, that glass breaking? That just means...the shattering of the human soul. Get the point?
Aside from that, they are very similar in nature. They both involve what almost seems to be a "dream-like" state with hysteria, and they both involve a child in one way or another. They both require you to bring your big brain to the table to try to figure out what the hell is going on (hence their "mystery/thriller" label). It's just that Don't Look Now requires you to be more abstract and patient.
Don't Look Now is much like Rosemary's Baby in that it involves a romantic couple, in isolation, with outsiders harming their trust and causing disorder and disillusion. Aside from that, I don't think you can compare this movie to anything else in the horror genre.
Last, if I had to give out a metaphor on the science fiction side, Triangle is to Total Recall as Don't Look Now is to 2001. You should be able to enjoy them both, but they do operate a little differently on the "gear" level. Triangle is more fast paced, with every scene urging you reconsider what you thought before, and it keeps you on your toes. Don't Look Now is more slow, gains it's momentum by building up characters, setting up the location, showing you bits and pieces of mystery elements, sometimes in abstract form, to keep you thinking. Then at the end you are given a "pay off", much like 2001, and you are left almost speechless while also being somewhat confused. Some people take this confusion and say, "that movie sucks." Many other people (like me) take the slight confusion and say, "oh man, I wanna watch this again so I can piece everything together."
Much like Triangle.
