You asked for it.
I can't resist to not just tell you my good and bad, but to evaluate all imortant characters.
I found it quite difficult to do that. Sometimes the filmmaker casted a very good and fitting actor, but then screwed up the script and ruined it (Pippin, Gimli). Then they casted a fitting actor who cannot act (Celeborn). Or they casted a good actor who doesn't fit in. And then they screwed up the script, too - out of habit (Frodo, Elrond).
So I gave everybody three categories:
Casting: Does the actor look like he should? Does he coincide with the book description?
Acting: Is he convincing? Does he act good or bad?
Writing: Do the writers keep with the book characters or do they change it? And if they do, does the character benefit or not?
Here we go.
Frodo C-
Casting: He simply isn't Frodo, is he? At least he looks like a halfling, so it could have been worse.
D
Acting: Not that convincing, but I can't point out anything particularly bad now.
B
Writing: The effect of the Ring is way to big. His attitude towards Sam is poor through TTT and half through RotK. The writers made him too much of a victim.
D
Gandalf A-
C: A brilliant Gandalf the Grey but less convincing to me as the White.
A-
A: Hey, it's Ian McKellen...
A
W: I don't like the Witch-King scene and his role in the Last Debate, but that's about it.
B
Bilbo A
C: One of the best choices of the whole film.
A
A: Nothing to complain about.
A
W: The quick aging bugs a little.
A-
Sam A-
C: Nothing to complain about.
A
A: I think Astin's acting decreases a bit at the end, but still:
B
W: Many changes were done to Frodo, but Sam reacts to them as the real Sam would have.
A-
Merry C-
C: They could have picked worse.
B-
A: You can see that Monaghan is not that gifted an actor, but still:
C-
W: Except for one scene during the Ride of the Rohirrim, he's portayed as a child. It's simply not Merry, but somebody else.
D-
Pippin B-
C: He's the oldest of the four hobbit actors, so he actually fits in age.
A-
A: Pippin bugs, but it's not Boyd's fault.
B
W: Most annoying in the first two (too childish, just for instance), but gathering ground in RotK.
D
Aragorn C+
C: A great Strider, but a horrible Elessar. Where's the kingliness?
C+
A: The lack of kingliness is not Mortensen's fault. I guess he acted just like he was told to.
B
W: He remains the scruffy ranger throughout the movies, he's close to usurping power in Rohan and I still can't believe he cuts the Mouth of Sauron.
D
Saruman B-
C: I think it was
alatar who pointed that out: Saruman looks dirtier than one expected him to.
B+
A: Solid, I would say.
B
W: In FotR he's captured well. In TTT we are made believe he's a mere puppet of Sauron. The short appearance in RotK was very uninspired.
C
Arwen C
C: I still think she looks beautiful and otherworldly enough.
B+
A: I guess it simply takes more than an average actress to play an elf.
C-
W: First she's an amazon, then she leaves Aragorn for the Havens on Elrond's word. I understand she had to be given more screen time, but was it impossible to fill that time with something that makes sense?
D-
Elrond C
C: Hugo Weaving may be otherworldly enough, but beautiful? He's surely not the way I imagine him.
D
A: Again, I can't point out what it is exactly, but he fails to convince me. Yet, being Hugo Weaving, his acting is nothing to complain about.
B
W: A true descendant of Thingol, isn't he? Elrond the statesman is captured quite well, but the private person? Not really.
D+
Boromir A
C: Though Sean Bean doesn't look like book-Boromir to me, there is something Boromirish about him.
B+ (A if he had worn a black wig)
A: No complaints.
A
W: Surely you could argue that a sympathetic Boromir just isn't a true Boromir, but without changing the character significantly the writers were able to add a lot of depth to him and made him accessible and identifiable with.
A+
Legolas D+
C: Orlando Bloom simply is too smooth a face.
C+
A: See Arwen.
C-
W: I can live with Legolas being a super-hero. What I cannot live with is his shallowness. Where's his fine humour? They cut every single sentence that made Legolas a two-dimensional character in the book. Now we're down to one.
E+
Gimli C+
C: No complaints.
A
A: Under all that plastics, acting cannot be easy. Yet he was convincing to me.
B
W: Let's just say that it's the scene with Galadriel that prevented an F.
E
Galadriel A
C: Since I'm preoccupied with a heavy crush on Cate Blanchett, I cannot give an objective grade. I think she looked awesome.
A+
A: If you take a great actress, you get a great elf.
A
W: She was slightly darker than in the book, but I liked that.
A
Celeborn D+
C: They could have picked worse.
C
A:
- Peter Jackson: "Marton, Celeborn means Silver-Tree. So that is what your acting should be like."
- Marton Csokas: "OK."
- Macalaure: "
E"
W: There's few you can do wrong with Celeborn. The one would be making him look like being second in power to Galadriel.
C-
Haldir D+
C: I think he looked alright.
B
A: Good actor: good elf. Average actor: bad elf. Below-average actor: oh, dear.
D-
W: The scenes in Lórien were alright. The scenes in Rohan were superfluous. At least they tried to make it realistic.
D-
Gollum A-
C: Serkis was great, as were the special effects.
A
A: See point one above.
A
W: The two sides of Gollum are made very clear, which is both interesting to see, as well as a bit disappointing, since he loses a bit of his scariness.
B+
Treebeard C-
C: The effects were good, but not overwhelming (his eyes disappointed me).
B-
W: The role was quite dumbed-down, as were all ents. The ent-moot was horrible.
D-
Éomer B+
C: No complaints.
A
A: Just like Monaghan, Urban is not the most gifted actor. But he acts well enough. It was a role made for him.
B-
W: Not having him at Helm's Deep was disappointing as this is where we get to know him in the book. The scenes we see of him before he meets the three hunters were very good on the other hand.
B-
Théoden B+
C: Not quite as I imagine him, but definitely not too shabby.
B
A: He's good, but not a straight A.
A-
W: He almost lets Aragorn usurp his throne. That shouldn't be. And deciding to hide in Helm's Deep was very out-of-character. But the burial of Théodred
was great.
B-
Éowyn A-
C: She could have been younger to me, but was perfect in all other respects - at least to me.
A-
A: See Théoden.
A-
W: She treats Merry like a child. But then, of course, Merry behaves like one. Her relationship to Faramir lacked about everything. Her scenes with Gríma were great.
B+
Gríma A-
C: He should look like a man of Rohan, but doesn't.
B-
A: Dourif makes the best out of the few scenes he has.
A
W: His hold over Théoden is far too strong. But the writers very well showed his wits, and I love the tear he cries when he sees the Uruk-hai army.
A
Faramir C
C: Sean Bean doesn't look like Boromir and David Wenham doesn't look like Faramir. The problem is, that, to me, Wenham doesn't have much Faramirish in him. I would say he was miscasted.
D
A: Yet, he does a solid job in the acting.
B
W: I share the opinion that the writers did not change Faramir that much. He only reacted to the changed Frodo like book-Faramir maybe would have. In RotK we get "our" Faramir back. Still he looks more like a victim to his father and his relationship to Éowyn lacked it all.
C
Denethor D-
C: He looks older than Denethor should look. In the book he tells us that he can still wield the sword. If he said it in the movie, we wouldn't have believed him. Isn't it ironic, that John Noble looks less noble than he should have?
C-
A: The budget of the movies must have run out just when it came to Noble's elvish lessons. Does he get a name right at all? Also, his acting looked very unmotivated to me.
D-
W: They could have done it so much better if they only showed Denethor looking into the Palantír. We aren't told about the cause of his madness, which also undergoes less development than in the book since Denethor is already quite mad in the beginning.
E
Nazgûl C+
The Ringwraiths were great in FotR, I think. But they were too unscary in RotK. They flew over Minas Tirith with nobody bothering to shoot them. The Witch-King almost defeats Gandalf. Instead of seeing his crown and eyes we get a ridiculous-looking helmet. And we don't get to see one at Cirith Ungol.
Orcs C
Lurtz was a great addition. The roles of Uglúk and Grishnákh were severely cut, much to Grishnákh's detriment. Cannibalistic orcs!? Sad they cut most of the talk between Shagrat and Gorbag. Gothmog and the orc overseer at Isenmouthe were both bad and unnecessary. Below the line, they were as bad and unsympathetic as they should have, but lacked depth. But many things lacked depth.
Imrahil F
C: How could this have happened? They seem to have forgotten to cast an actor for him! There was too much money in this project for such amateur faults. Unexcusable.
F
W: Seriously now. Yes, many characters were cut from the book and maybe it's a matter of taste who the worst omission is. To me it's Imrahil, the model Gondorian, since Faramir is out for most of the RotK and Aragorn is (well, should be that is) a bit untouchable.