You can move through a square occupied by a friendly character, unless you are performing a Move By or a Move Through. When you move through a square occupied by a friendly character, that character doesn't provide you with cover.
You can't move through a space occupied by an opponent who is not helpless (for example, unconscious or paralysed) unless the opponent is the target of your Move Through. You can move through a space occupied by a helpless opponent without penalty. Some characters, particularly very large ones, may present an obstacle even when helpless. In such cases, each 1m of their space you move through counts as 2m of your movement. You can't end your movement in the same space as an unfriendly character unless it is helpless.
Very small characters with at least 2 levels of shrinking can move into or through an occupied space. The character provokes attacks of opportunity when doing so.
Any character can move through a space occupied by a character at least 3 Growths larger than it is. Conversely, any character can move through a space occupied by a character at least 3 Shrinks smaller than it is.
Note that some characters break the above rules. A character that completely fills the space it occupies cannot be moved past, even with Acrobatics or similar special abilities.
Difficult terrain hampers movement. Each metre of difficult terrain counts as 2m of your movement. If your space contains different kinds of terrain, you can move only as fast as the most difficult terrain you occupy allows. Flying and desolidified characters are not hampered by difficult terrain.
Like difficult terrain, obstacles can hamper movement. If an obstacle hampers movement but doesn't completely block it, each obstacle or obstructured metre counts as 2m of your movement. You must pay this cost to cross the barrier, in addition to the cost to move into the space on the other side. If you don't have sufficient movement to cross the barrier and move into the space on the other side, you can't cross the barrier. Some obstacles may also require an Acrobatics skill check to cross. On the other hand, some obstacles block movement entirely. A character can't move through a blocking obstacle. Flying and desolidified characters can avoid most obstacles.
In some cases, you may have to squeeze into or through an area that isn't as wide as the space you take up. You can squeeze through or into a space that is at least half as wide as your normal space. Each 1m of movement into or through a narrow space counts as if it were 2m of your movement, and while squeezed in a narrow space you take a -2 penalty to OCV and DCV. A character can squeeze past an opponent while moving but it can't end its movement in an occupied space.
To squeeze through or into a space less than half your normal space's width, you must use the Contortionist skill. You can't attack while using the Contortionist skill to squeeze through or into a narrow space, and your DCV is reduced to 0.
Sometimes a character ends its movement while moving through a space where it's not allowed to stop. When that happens, you stop in the last legal position you occupied, or the closest legal position, if there's a legal position that's closer.
When your movement is hampered in some way, your movement usually costs double. For example, each 1m of movement through difficult terrain counts as 2m of your moevment. If movement cost is doubled twice, then each metre counts as 4m of your mvoement. If movement cost is doubled three times, then each metre counts as 8m of your movement, and so on.
Despite penalties to movement, you can take a full-round action to move 1m in any direction. (This rule doesn't allow you to move through impassable terrain or to move when all movement is prohibited.)
Characters with more than 2 levels of Shrinking or with 1 or more levels of Growth have special rules relating to position.
Very small characters take up less space. This means that more than one such character can fit into a single space, even if they are opponents. Four characters with 2 levels of Shrinking can fit into the same space as a typical adult human. Twenty-five characters with 3 levels of Shrinking or 100 characters with 4 levels of Shrinking can fit into the same space as a typical adult human.
Characters with 1 level of Shrinking typically have less than 1m of reach, meaning they can't reach into an adjacent space. They must enter an opponent's space to attack in melee. This provokes an attack of opportunity from the opponent. You can attack into your own space if you need to, so you can attack such characters normally. Since they have less than 1m of reach, they do not threaten the space around them. You can move past them without provoking attacks of opportunity.
Very large characters take up more space, and typically have additional reach. This means they can reach targets who aren't adjacent.
Targets who are behind cover are harder to hit. In addition, you can't execute an attack of opportunity against an opponent who is behind cover.
To determine whether your target is behind cover from your ranged attack, choose any corner of your space. If any line from this corner to any corner of the target's space passes through a space or border that blocks line of effect or provides cover, or through a space occupied by a character, the target is behind cover.
When making a melee attack against an adjacent target, your target is behind cover if any line from your space to the target's space goes through a wall (including a low wall). When making a melee attack against a target that isn't adjacent to you (such as with Stretching), use the rules for ranged attacks.
A low obstacle (such as a wall no higher than half your height) provides cover to characters within 10 metres of it. An attacker can ignore the effect of a low obstacle if they are closer to the obstacle than the target.
You can't take an attack of opportunity against an opponent you cannot locate with a targeting sense, even if you know what square the opponent occupies.