A bit late, but as promised, here’s the assassin rant.

1) Your character is not special or important just because she’s an assassin.

This ties back to the special abilities rant, the characterization rants, and a lot of others on the protagonists. Fantasy authors seem to have the persistent idea that if they make their character a particular profession/give her a certain ability/give her a certain trait, then she’s Unique, and they can skip all the hard work of characterization.
That kind of thinking is actually more likely to fall into the shit pit than escape it. Create an Assassin instead of an assassin, and she’ll be just like every other Assassin out there. Her “emotionless eyes staring into the depth of his soul” won’t be anything new. Nor will her tendency to wear black, or be jaded and empty, or have other people fear her. All are “characteristics” of assassins.
Don’t base your assassin character’s relationships with others, part in the story, emotions, history, personality, and so on solely on the fact that she kills people for money. Just because you’re impressed with her doesn’t give anyone else a reason to be. She also has to be a person that others will be impressed or fascinated or disgusted by, someone they’ll enjoy reading about.

2) The “thrill of the hunt” assassins are nothing new at this point.

The Evil Overlord List says it all, really: “I will only employ bounty hunters who work for money. Those who work for the pleasure of the hunt tend to do dumb things like even the odds to give the other guy a sporting chance.”
This is a cliché that’s really, really hard to do well. Why? Let me count the ways:

  • Most fantasy writers, again, are convinced that merely writing the idea down makes them cool and original. No, it doesn’t. You have to work to put your own spin on the idea.
  • The author gets more and more obsessed with writing the story as a contest between the assassin and the person he or she is trying to kill. That format is a lot like the arranged marriage story: there are basically only two ways it can end, and the audience can see either coming.
  • The assassin is supposedly the kind who can laugh and joke and then stick a knife into people. Yet the humor is often extremely forced, and the assassin acts more unbalanced than humorous. How is someone that crazy supposed to be a character who can even the odds and yet still win?
  • The author is romanticizing murder. This happens a lot with assassin characters anyway, but it’s all in the presentation; an assassin complex enough can at least make the audience understand his or her perspective. With the thrill of the hunt killers, the author is asking us to think that, awww, somehow it’s all right, because the motive isn’t the same. Hunting for pleasure is better than hunting for the money you get for it? How?

The thrill of the hunt killer thrilled me exactly the first time I saw it, in Terry Brooks’s Shannara quartet. I can’t think of one since then who’s convinced me that the author wasn’t just saying, “Look how cool I am!”

3) Just because the character is an assassin does not mean that he or she is an awesome fighter.

I would actually expect a mercenary to be a lot better, because usually he or she is experienced in more terrains, has served in various situations (with a company, with large armies, in raiding bands), has fought in rough conditions like mud and rain and so on, has armor, and is allowed to be as dirty and rough-and-tumble a fighter as an assassin. Typical fantasy assassins are the ghouls of cities and palace corridors. Ask them to pursue their prey into a forest in the middle of a downpour, and I don’t know if they’d have a clue what to do.
Yes, a fantasy assassin may know how to use fairly uncommon weapons—a throwing knife, a garrote, a stiletto. But they actually have to be able to get close to use most of those. And even then, they still have limitations, just like swords against ranged weapons. Throwing knives can slow a person down, but often don’t kill. Garrotes are fucking hard to use if the enemy is bigger than the assassin; it takes a lot of work to strangle someone. Stilettos are precise weapons, but they’re not really any deadlier than, say, a rapier or a broadsword. Unusual weapons might make an enemy facing the assassin fumble, but they wouldn’t guarantee a kill.
Also, training someone to sneak about, walk on tightropes, use poison, hide in shadows, and all the other “assassin craft secrets” sound a lot like training someone to murder to me. They don’t sound like training someone to survive an open fight against an armed and aware enemy. And it certainly doesn’t sound as though an assassin would be able to kill a rich target every time. Even if his fee is high because he’s very good, if he’s going up against a king or rich noble, why wouldn’t they be able to hire bodyguards as good?

4) Work out the framework for acceptance of assassins.

Okay, so assassins in your society are tolerated. They probably have a guild.
Why is that? People have even less reason to tolerate assassins than they do thieves. And the idea that everyone is scared of them just won’t fly. Once again, I say to you: powerful mage, big fireballs cast from a distance, bye-bye charbroiled assassins’ guild.
The best fantasy examples (once again taken from Pratchett and Brust) allow the operation of assassins within severe rules. Pratchett’s assassins are given an education and earn very handsome fees for their work, which means that, say, Joe Blow just can’t hire them to kill someone who killed his brother. Brust’s operate within the organized crime structure of House Jhereg, have different levels of “work,” as they call it (“revivifiable” in cases where the death is mostly a warning and the victim can still be brought back to life, permanently dead with the brain or spinal cord damaged for a bigger crime, and Morganti, or soul-destroying, for when the victim’s done something like betray House Jhereg), and will get the Dragaeran Empire on their ass if they kill the wrong person. There’s none of the nonsense that exists in some fantasy books, where assassins walk around openly and no one ever, ever tries to harm them; they just bow and look awed.
So your assassins should not make a practice of killing everyone in sight, and have fairly strict rules that they have to follow. None of this “Assassins rule the world from the underground!” business, especially when they’re portrayed as fighters that sneak around in shadows, and nothing more impressive.

5) Reconsider the “assassin’s mark.”

This is the mark that supposedly every assassin wears to tell other people that they’re assassins. A skull and crossbones, or a dagger, or the coiled rattler of the Secret Desert Assassins, or whatever.
Reread this part: to tell other people that they’re assassins.
Is this something you want other people to know? As an intimidation tactic, maybe, but I fail to see what’s intimidating about the hero checking over the dead assassin and finding his affiliation tattoo somewhere. The assassin is the one who’s died, not the other way around.
It’d be better to have a secret sign, if you must have one, that only assassins knew about, and that wasn’t on the body, say a gestured language or a verbal code similar to thieves’ cant. Yes, the chance still exists that an outsider would find out about it, but it’s much less than the chance that someone would see the OMG-secret snake tattoo on someone’s wrist and blurt out, “You’re an assassin!”

6) Know what is fatal.

One of the most famous assassin characters in fantasy is probably R. A. Salvatore’s Artemis Entreri. Several years ago, I read a story about his childhood, “The Third Level,” in which he killed a greedy mentor while he was still a teenager by putting ground glass in his food. This supposedly proved he was a lethal assassin as a child and all kewliez and stuff.
There’s just one problem: Ground glass is not the fatal killer everyone assumes it to be.
Some of the things, like this, that authors assume fondly are assassin craft tricks just don’t work. As another example, think about the training that gymnasts have to go through in order to be able to perform as well as they do, and what kind of body type they usually have. Now imagine a tall and lanky fantasy teenager who’s just started training as an assassin, has spent most of her time learning how to handle weapons, and can supposedly flip in and out of windows with the same grace and skill that an Olympic-level gymnast has.
Riiiight. She’d be more likely to fall to the floor with a noisy clatter of steel.
The same thing applies to walking silently. In slippers, on carpeted floors? Not a problem. It will take quite a while to convince me that your assassin, who has steel spurs in his boots to aid him in climbing, and who is walking around on loose roof tiles, will not cause anynoise to people listening below.
Assassins are probably even worse than thieves for doing impossible things because they’re “trade secrets.” Why not just go the whole-hog and make them a kind of magic that only assassins know how to harness? Telling me there’s a spell of grace on the teenager may not solve everything, but it would be easier to believe that than to believe she could move like a gymnast after three days of training.

7) Think through the implications of murder.

What will it do to someone’s personality? If you can believe fantasy authors, it has only two effects: to make someone in love with the thrill of the hunt, or to make them “emotionless” and jaded and angsty and one step away from writing bad goth poetry.
It doesn’t have to. Remember that perfectly ordinary people in the history of our world have tortured, maimed, raped, robbed, and, yes, murdered other people. Your assassin may have other interests. (Terry Pratchett’s assassins do; his Patrician Vetinari was once an assassin, and yet doesn’t spend all his time in the shadows plotting to kill people. Brust’s Vlad Taltos is an assassin, but he’s also a skilled witch and sorcerer, and has a wife, and gets involved in politics). What are his relationships with other people like, the ones who may not know he’s an assassin? Is all that joy really feigned? Does he do nothing when he’s not killing but stare at the wall, or write things like

the stars are slanting down on me
the dark is closing in
blood of the pain of my soul
cuts on my hands
woe woe to the underland

all the time?
An assassin who starts having a crisis of conscience about his job (which also happens to Vlad Taltos) could be a really interesting character. So could one who just goes out and does his job, and then goes home and plays with his kids. That latter one would be the most likely scenario in a world where assassins are as common as fleas, the way that some authors write.
Really. They don’t have to be killing robots, or emotionless robots, or angsty robots redeemed by LOOOOVE. They can be people, too, and deal with what they’re experienced in other ways.
D&D stereotypes next, I think. Or why fantasy is not a role-playing game.