With state changes, characters take on the appearance of having senses. Which explains why sound and vision are two of the most common triggers outside of combat. Sound triggers are activated when the player makes a certain amount of noise within a certain distance from an NPC. Vision triggers most often happen as a cone of sight in front of an NPC. If you wanted, a smell trigger could function in the same way as either or a combination of both if you want REAL odor physics. Obviously touch and taste are much closer to combat, needing a prompted, physical trigger.
Sound is certainly a staple trigger in stealth games. Nothing makes a cat and mouse game come alive more so then when an NPC reacts to a sound and enters an alerted state. Sight of course follows the same function, be it moving through the vision cone of an enemy as you slide between cover or leaving an unconscious enemy out in the open to be found. I can't think of any stealth games that use smell and odor, though the amount of trash bins, barrels and sewers the player hides in, one would think the stench could be a factor. As silly as that sounds, hunters use all manners of scent blockers and odorless soaps to camouflage themselves, so it could certainly be added and tracked. In a stealth game that is all about the player using all their senses to hunt their enemies, giving some of those senses to those being hunted is a powerful piece of player/game connectivity. Giving a character a sort of sixth sense would be done by proximity triggers. This is when the player enters into the personal radius of an NPC. It could be the same as their sound radius, but instead of noise triggering the state change, just crossing into the volume would trigger it. When you encounter a Big Daddy in BioShock, dutifully following a Little Sister, the moment you step within a certain proximity of them, the Daddy has a state change. He stops and menacingly turns towards you, like a mother bear, letting you know if you get any closer there will be repercussions. Such a small, simple moment, but incredibly powerful. The trigger matches the personality and purpose of the character. The goal of the Big Daddy takes on a specific objective with this state change, from just generally protecting their Little Sister, to now protecting them from you. The Legend of Zelda: Skyward Sword uses a proximity trigger in a wonderful way as well. There is a marketplace vendor in the Skyloft Bazaar that excitedly jumps up and down as you approach his stand. Seeing him get so excited at the prospect of you buying his goods adds a sense of life anyone with retail experience can relate to. But the added spice was when you walk away, and he instantly deflates and turns his back as he walks away utterly destroyed that you didn't buy anything. With only a couple of animations and a proximity/visual trigger, that NPC became more empathetic than a lot of main characters. While the animations are fun, it is the trigger and state change that makes him come alive by allowing him to wear his heart on his sleeve. Obviously transitional animations play a big part in these state changes, as those are the moments that the NPC is having the actual change of emotion or thought process, but if those transitional animations aren't set to an appropriate state change, the illusion of life can break down quickly. Pathing You can create the most amazing walk or run cycle, full of personality and weight, but if the character walks in an entirely linear and uninteresting path, it will all have been for not. Pathing is something most animators never pay much attention to, as its implementation happens by any number of designers or programmers throughout various levels and can change at the drop of a hat. In fact, focusing on specific pathing of a character in every instance would be an incredibly large undertaking. But something far more manageable is thinking about HOW a character will path when you deliver the suite of animations. How a successful General of an army walks from place to place is going to be different from the drafted recruit freshly thrust into a war. The General may very well walk from destination to destination without stopping. But a new recruit will very often stop to look around or slow down/speed up depending on what is around.
Pathing is also a great way to add some awareness and appropriate traits to animals and creatures. Watch how a spider walks. Not just the cycle of their legs, but how they path. They will walk for a bit, then stop. Often for a significant amount of time. Then they will just take off in another direction. Stop for a brief moment, then continue on. This staccato movement is as significant towards creating a fully realized spider as how the animation looks. A complex animation matrix of turns, stops and starts aren't even necessary. All you would need is to inform the designer that places the pathing throughout the level that long, linear paths is out of character. What if we add state changes into the pathing? Adding in points of interest or interactables certainly add work to the plate of everyone involved, but they are also powerful additions towards creating interesting and appropriate pathing options. If throughout a level each AI has one or two objects that can grab their interest, you can use those as triggers for a state change when placed throughout the AI's path. This gives the character a little bit of business in the world, making them feel like they are there for more then just the player's use. Let's use a rabid canine walking across a wasteland as a test case for all of these pathing options. Or a hopped-up rabbit that loves the smell of poppies in the spring if you want something a little more light-hearted. In both cases, their movement is certainly going to be erratic, so pathing that uses sudden directional changes would be best. Now let's say there are random animal carcasses littered across the landscape or poppy patches in the case of the rabbit. Whenever one of these objects is inside of the path of our furry friend, the object triggers a state change that has the elated mammal jump into the middle of it all, rolling around like a pig in filth. With this, a myriad of player possibilities take place. The player could run or engage while it is distracted. When being chased, the player could lure the creature away by putting one of these objects between them. As a designer, you could even tag these objects with a buff that emboldens the creature after it has satiated its desire. Layering animations is also a powerful tool when it comes to pathing. In life, a person doesn't just go from one action to another, completely finishing one before moving onto the next. Imagine you are walking towards a door that is slightly ajar. You wouldn't walk up to it, stop, push it open and then walk through. You would do that all as one continuous motion. Giving moments like that to a character is what makes them seem aware. Layers are a way to achieve this. Adding in a look at control to those objects of interest helps to show a character is interested in something without breaking their flow. Uncharted 3 uses layers to have Drake's hand reach out and touch a wall as he passes by, helping to cement him in the world. It also fits his personality that as someone who is always tripping and falling down that he would want to continually balance himself against a solid object. This also requires IK on the arms, so it is most likely too tech expensive to be used on NPC's, but it is something to think about it. As you can see, the added benefit of thinking about pathing as it relates to characters is that it varies the gameplay as well. And if that pathing truly matches the intent and personality of the character, it shouldn't be dissonant with their design. In a stealth game, those varied paths make each class of character a unique experience from one another. In a game where you try to control the amount of enemies you take on at one time, this variable keeps you on your toes as to when and where to engage in a play space. Pathing, and the elements and obstacles you place within that path, are what define a character's place in the world and how the player perceives it. Animation doesn't stop at export Just like film animators follow their shots throughout the pipeline, making sure everything matches the performance laid out, we need to follow our animations throughout the game. If you apply all the thought and energy you instilled into the character into their implementation, you will find game animation can create as memorable characters as any film. [This piece was reprinted from #AltDevBlogADay, a shared blog initiative started by @mike_acton devoted to giving game developers of all disciplines a place to motivate each other to write regularly about their personal game development passions.]