Watching the film again I have to say it would not be hard for me to put Sergi López as co-lead in the film. This is due to the fantasy side of things being told from Ofelia perspective, but the scenes of reality being mostly told through the eyes of Captain Vidal. Vidal in addition to being the head of the military base he also is Ofelia's stepfather having been the one who impregnated her mother. Although Captain Vidal is the most prominent character on the reality side of things he might as well be the big bad wolf since Vidal is written to be a personification of an organized evil, usually even in the cases of this type there may be a bit of humanity found, after all there is some even in the case of Ralph Fiennes's Amon Goeth in Schindler's List, that's not really the case for Vidal. Even in his personal life Vidal shuts down any discussion of those things known as feeling and the only thing he seems to care about is the birth of his son, but mostly as though it is a possession more than anything else.
As written Vidal certainly could have been extremely over the top and almost ceaseless in his obvious evilness, but to his credit that's not really the way López plays him. The few chances he gets to explore any other side of Vidal López tries to make something out of them. This includes the brief moment where he greets his wife as López gives a nicely feigned charm although this is shortly undercut by his particularly rude treatment of his Ofelia. His unpleasantness at the dinner table where he ignores all personal questions is given some sense by López suggesting more of his uncomfortableness with interpersonal interactions than simply evil motivating him. There is also the running idea of his father's watch which originally had the exact time his father died, and he was some how given the message to show him how a brave man dies. The whole watch idea I feel is a bit underdeveloped in terms of the writing and López can't quite make up for it. He's not bad in these scenes, but it never has the impact that direction seems to imply that it should have.
The main point of his performance though is the scene of Vidal being a murderer and torturer of rebels, their conspirators, and anyone unfortunate enough to be accused of such. López is very good in these scenes by not playing them as a crazy psychopath but rather an official soldier of death. López plays Vidal as particularly relaxed in these scenes as though this is when the man is truly in his element. López does not scream or yell but rather has such a viciousness simply in the way he casually way he portrays his manner in the killings. López brings such a comfort in Vidal as he takes his hammer to men, or shoots them down. It is not that of a man seeking gratification really, but rather as a man going about his job in the fashion he knows best. López makes the character the most cruel by portraying the efficiency of his method at all times. López, if he must be the big bad wolf certainly earns the title. As good as he is in these scenes I do feel something is missing and there may have been a little more to explore with the character. López does find some depth but in the end the character still feels fairly simple. He's certainly never lacking in the needed menace for the character, but I could never shake the feeling that something is missing.