Sorry to state the obvious, but novels are not a visual medium. When a character is described as unattractive, we take their word for it, and create a picture of them in our minds with that in consideration. Because beauty is a personal and subjective thing, many people will have differing ideas on what is unattractive. Not to mention, if not a lot of detail is given, you can interpret 'unattractive' to mean anything from not conventionally attractive in the time period the novel is set in, to utterly repulsive. But what happens when a novel is adapted into a film? Casting directors need to cast the beautiful characters, which should be easy enough with Hollywood's abundance of gorgeous stars. But what to do about the plain women and the unappealing men? Will they cast an unattractive actor? Perhaps a plain, or less conventionally attractive actor? Unsurprisingly, they will often opt for an attractive actor, and they may or may not go to lengths to hide their good looks, depending on how important the character's appearance is to the plot. But expect the character to still look beautiful, as a quote from The Simpsons of all places puts it quite well, they will be "TV-ugly, not ugly-ugly".
It always bothers me when a character in a book is described, sometimes even at great length, as unattractive, at least conventionally so, and yet they are played by a beautiful actor. One example that perfectly illustrates my point is Keira Knightley as Eliza Bennet in the 2005 adaptation of Pride and Prejudice. It was a stupid casting choice because Knightley is not only a very attractive woman, she probably would have been considered very attractive in the era in which the film is set, so you can't even justify it with the differing standards of the time periods. The casting of Jennifer Ehle in the 1995 BBC adaptation was, I felt, much more fitting. Although by no means an unattractive actress, I would believe that, with her rounder face and softer features, Ehle would not have been considered very pretty by the standards of the time period, while her sister Jane, played by Susannah Harker, had the more desirable long face shape and sharper, more striking features.
It is very common for films to have roles of supposedly unattractive characters played by attractive actors, simply because... um... actually I don't know why. Are film makers afraid that we as viewers won't watch a film if it has realistic or ugly characters in them? If we could handle Lon Chaney's hideous makeup in the 1925 film The Phantom of the Opera, we can handle a genuinely plain person every now and then. Anyway, for whatever reason, beautiful people are cast in these roles, usually 'de-prettified' in some way by dressing them in less flattering clothes, giving them bad hair or by adding glasses. Admittedly, giving an attractive actor unflattering clothes is a fairly good trick, but it doesn't really work because a beautiful actor is still beautiful with messy hair and a baggy argyle jumper.
Now, there is something that I must address, because if I don't it is sure to come back and bite me in a less than pleasant place. Here I am going on about characters having makeovers in film adaptations, when I thoroughly enjoyed the film adaptation of Louis Sachar's Holes, despite the fact that this film is guilty of just that. In the book, the protagonist, Stanley Yelnats, is an awkward teenager who is frequently teased for his weight, and later given the nickname Caveman because of his size, and also because a fossil that he finds. In the film, Stanley is played by Shia Labeouf, a tall, thin, lanky actor who is, let's face it, very attractive. Having watched some of the film's extras, they do explain that they had considered casting a chubby or overweight actor, but because in the book the character becomes slimmer and fitter over a long period of time, which would have been very difficult to show in a film, they had to remove that aspect of the character. They also mention that chubby teenage actors are hard to find, although I doubt that very much. True, this casting choice did mean that the whole bullying thing had to be swept under the rug, and the Caveman nickname didn't work quite as well, but I understand why they made this decision, and at least they put some thought into it and didn't just think "well, we can't cast an actual fat person, our characters all need to be attractive!" and I thought the film worked well despite this, so I will let it slide because I just love that film.
Back on the subject of glasses, I have to say that something I always take issue with is this persisting idea that glasses are supposed to be unattractive. It seems that, in movie land, people can't see Julia Robert's face behind a pair of glasses; maybe they are magical or something. Adding glasses works somewhat in older films because glasses were not particularly stylish, and they didn't come in very many differing or fashionable designs, so in these cases I will let it slide, albeit reluctantly. But the problem is that it is still used as a 'de-beautifying' technique to this day, and as a glasses wearer, I can't help but feel offended at the implication that my choice to not stick my fingers in my eyes every morning or spend hundreds of pounds to be shot in the eyes with a laser makes me 'frumpy'. On top of all of that, it is just lazy. Some films utilise makeup and prosthetics to make their actors look less conventionally attractive, but some just feel that throwing on a pair of glasses will do just as well. Don't expect us to see a pair of glasses and automatically see the character as unattractive. Christopher Reeve was still Christopher Reeve, even with his Clark Kent glasses on. Now, I know that Superman wore glasses as a disguise and not as something to make him look less attractive, but my point is that, even when he wore large, thick-framed glasses, he was still a good looking man.
At least the idea of the 'brainy specs' makes a little bit of sense, considering short-sightedness is related to reading a lot of books, even if the reason for this isn't fully understood, so I can accept that it is somewhat grounded in logic. Also, at the very least it is a somewhat flattering trope, even if it does have a somewhat patronising feel to it, as if to say, "Hey, you four-eyed kids may not be good looking, but at least you're smart!" although I suppose it is to separate the 'brains' from the 'beauty' for the sake of balance.
Incidentally, many people now consider glasses to look very attractive, so suck on that, Hollywood.
