Pacific Rim has giant robots manned by two pilots who are in each other's heads through a tech called "drift." No clue how it works. Don't care. They "drift" and the two of them are then able to act as one and control one of the giant robots, the Jaegers (German for "hunter") that are used to defend the earth against the Kaiju (giant monsters - think Godzilla as the prototype ultimate Kaiju). The Kaiju don't come from outer space, they come from under the ocean... only they DO come from another plane(t) through a rift under the Pacific Ocean. For Kaiju that's a trope and it's honoring that trope to have them from the ocean. Who remembers the words to the Godzilla cartoon from '78 theme song?
So, we've got giant robots that fight the invading Kaiju that, unlike the Godzilla Kaiju aren't a result of nuclear accidents or nuclear testing and part of the scare the pants off the public about radiation and/or nuclear weapons/energy as the setting in which our movie happens. I say it like that because the movie isn't the setting in this case... close, but not completely.
Evidently one of the actors (other than Ron Perlman) was someone before this movie. I didn't recognize anybody but that's because a) I'm terrible at recognizing actors and 2) because I don't watch Sons of Anarchy. The characters had actual individual arcs within the movie where no character ended the movie in the same place they started it (Sometimes it was because they were dead... but in one case I was all for that so yeah.) There was personal growth. There was back story. There was slow reveal of character motivations that made me interested in the characters. Now, to be fair... remember this is in the context of a movie about giant robots beating up giant turtle monsters from outer space that crawl out of a crack in the ocean. That being said, they did a good job of character building, character development, and having the characters be interesting. (Exception: The Russian couple. They were sort of cardboard cut-outs and I get they were ancillary, but they seemed the most 1 dimensional of the group... in a 2 hour movie that's not bad really.)
Comic relief provided by two scientists forced to be together in spite of radically different... think Odd Couple scientists with wildly varying disciplines. They don't like each other but secretly they do... and they wind up helping each other out in a completely different (almost) story arc, B story it'd be on a TV Show, that only tangentially relates to the primary story line but is interesting and fun at the same time. They served as a sort of comic relief to help build tension between scenes. They were the time you'd get to sit back in your seat and relax... only they didn't stay that way. They wound up with their arc being exciting as well, "This isn't a shelter! It's a buffet!"
I mention the relatively unknown actors because I like that in a movie. Using all A-list actors to me brings a certain amount of baggage to a movie. Tom Hanks, Tom Cruise, Leonardo DiCaprio, Brad Pitt, when you hear those names you know when they walk on screen that a) they're the star, b) they're safe & nothing will happen to them and c) they're probably going to be a good guy or heroic or likable. (Leonardo almost didn't make the list because he, to me, is a better actor than either of the Toms and plays a wider range of characters better than the other two do. He did Django as a complete bastard really really well but I had to include him so I didn't have just Toms on my list.) So, I like when big movies don't use a lot of big names or DO use big names in small parts. Ron Perlman as the gold shoe wearing eccentric dealer in Kaiju parts on the black market was great casting.
I'll talk about the movie and what I liked about it all day if you let me so I'll stop. Go see it. It's a lot more fun than you're giving it credit for. It's not just that it's better than you think. It's better than it has any right to be.