Shabbat 107 - Two people catch a deer
If a deer was inside the house, and a person sat in the doorway without completely blocking it, so that the deer can get out, that person is not liable for trapping the deer on Shabbat. However, if a second person sat in that door and made the exit completely blocked, the second one is liable. The first one is not, because he did nothing.
On the other hand, if a person sat in the door and completely blocked it, and then the second one came and sat there also, then, even if the first person leaves and the second one is the one who is blocking the door now - only the first one is liable, because he effected trapping, while the second one again is doing nothing at all.
The eight reptiles mentioned by the Torah are different from other crawling creatures: if one bruises one of them (such as a mouse) on Shabbat, he is liable, but for others (like a snake), he is not. The difference is that a mouse has thicker hide, and a bruise leads to blood accumulation, which is taking away a life, albeit in a small measure. The hide of other reptiles is soft, akin to skin, and the reddening caused by a minor bruise soon dissipates.
One is also liable to trapping one of the eight reptiles, because people do normally catch them, for various reasons.
Art: Rosa Bonheur - Two Deer In A Forest Glade