Depends on what characters being stupid you mean, with a few of them it is done deliberately so to prove a point.
Maiya and Kiritsugu.
When Iri managed to tie Kirei to the tree, she called Maiya to shoot him. Now I'd understand if Maiya was lying far from the gun and due to her injuries she could not crawl fast enough to get it before Kirei got out. But she later showed she had a gun on her that she could swiftly pull out. So what did she do? Lying there being in awe of Kirei's...destroying the tree in three hits...The way to solve this is absurdly easy. Make Kirei destroying it in one hit/one second and use his speed to hit Maiya before she could do anything. Or doing the ''time-stop'' trope where us audiences see it as something long in real-life time but in-verse it happened in a fraction of second by putting a clock on the screen to show that or maybe the characters go black with only the outline of their body in white.
As for Kiritsugu...if he can shoot people then why not at the brain? A seasoned veteran killer cannot aim well enough with a distance of barely more than 10 meters? Especially when he has a fucking Triple Accel...
Assassin´s name was given.
Oh, but now that I talk about this, the show doesn't actually ever explain the Servants' origin well to give full context of their characters. Lucky I know all of them except Assassin. I can understand Excalibur-King Arthur and Alexander the Great are well-known and maybe Gilgamesh but Diarmuid? O__O
Also is Saber lesbian or bisexual (I don't get this impression from what I saw on Internet regarding this Fate universe)? How does the original story of Lancelot having affair with Queen Guinevere works when he knows Saber is a woman? Did he betray before he knew? It's also a pretty damn waste IMO when this stealing boss's wife business happened to Lancer, too, and with all their interaction together, this is never raised.
Character investment, you are right except Emiya for me, he was a tragic figure i really felt sorry for.
And Rider, dude just stole every scene.
Emiya and Rider belong to the second category where I love their stories, background and what Uroubuchi wants to convey through them but not themselves. Basically I like what the vehicle carries and not the vehicle itself. Though it can be argued they are one entity in a sense, I just don't feel much for the characters. I just don't. Can't express it well but when Emiya shot his daughter and strangle Iri in the Grail's illusion I literally don't feel a thing for him. Not even a sense of sadness like I would feel for my favorite characters in other manga. Not sure if my sympathy function works on a hit-or-miss mechanics or something...And I have no attachment to Rider. But yeah, he has the best acting out of the cast, I think. At least as far as fully showing the point of his character.
The thing is due to no to little attachment as well as the fact that many of them are mouthpiece, Gilgamesh and Rider stand out as the ''not serious'' guys who lift atmosphere whenever they show up.
Oh, I do like Caster team When you get tired of justice talk, it feels like fresh air. I think I'm too desensitized by this medium...
We can talk about it in-depth if you are really interested, if not it´s also ok if you want to take away what you ahve already understood.
Well, I'm not surprised when utilitarianism makes its appearance again in an Uroubuchi work LOL. He sure puts a lot of ideas in this piece. It's not an uncommon structure by any mean, if not too common where you have a battle royale and each participant has his/her own story that has its own point/meaning. But what's great is Uroubuchi managed to make all of them important in the grand scheme of things, a great part thanks to the screen time being divided smartly, and characters who are minor still stand out by having a distinct feature (like Aoi and the bug vampire old man). Though, again, it's regrettable that a lot of these ideas have been brought to us through talkfest.
The themes and ideas that I can see (well, some of them is too apparent) from my first-watch is:
-Utilitarianism through Kiritsugu
-The true/right way of king/monarch through King Banquet and each of the 3 is worth a topic (though maybe not Gilgamesh... )
-That whole honorable knighthood and its impracticality through Lancer's death scene and Kiritsugu's dialogue.
-Parenting through Tokiomi and Father.
-Psychopath through team Caster and Kirei.
-Self-discovering and self-worth through Kirei and Wager
-Selfish and myopic love through Kariya (it's not just he and Aoi but also he and Sakura).
-Family through Kiritsugu and Tokiomi.
-The Holy Grail's mechanics of granting wish is also interesting. From what I understand, it is indeed omnipotent but it has a condition: it only grants the wisher what he must be able to comprehend/imagine. Also it must be his most genuine wish which is why it is revealed Kiritsugu cares so much for his family (he ends up killing them anw but I wonder if it's because he knows they are illusions? Uhm, maybe he would do that in real life, too but before the war started he actually got hesitated and asked Iri if they should abandon the fight which is consistent). The whole War is the ultimate troll.
-Free will vs tradition through Tokiomi and Kariya
-Kayneth and Lancer's situation as well as Saber and Berserker's worth story on their own.