Hermit crab behavior can vary signifcantly from one individual to another. If you own several, I am sure you will have recognised that fact that each individual has its own personality and traits to go with it. A good indication of whether you are buying a suitable hermie for a pet is to observe the hermie's personality and habits when it is inside its shell. Now I know this sounds a bit dumb, but you can tell a lot about a hermie by how he reacts when he is placed on your upturned palm. If he comes out fairly quickly then the signs are good because he is obviously quite curious and relaxed around humans. If he comes out after a while, making clicking noises, then he is curious, yet frightened, which is not a bad thing. However, if he comes out clicking and tries to pinch, then he is aggressive and probably would take a lot of taming in order to make a good pet.
Another way to guage the hermie's personality is by introducing them to other hermits and seeing how they react. A friendly hermie will use his feelers to gently touch another hermie's feelers whereas an aggressive hermie will lunge towards the doorway of another hermie, spoiling for a fight. It is, however, normal hermie habits and practices for a large group to be observed pushing and climbing over and under each other. This is more like playing and is pretty harmless behavior.
Digging and climbing are normal hermie practices both in the wild and in captivity. It is important, therefore, to create a home from home and ensure that your crab has sufficient sand in which to dig and that he has a supply of rocks, shells, wood and plants in which to climb and explore. If your pet hermie seems to be digging feverishly, then he may well be about to molt which again is normal crab behavior and vital if he is to grow in size.
Nocturnal activites are very much part of normal hermie practice and go a long way to explaining how a neat and tidy crabitat is totally trashed by the morning! Hermies tend to remain fairly inactive during the day and may well emerge on mass around sunset. If you keep several of the crawly critters it can be fun to watch all of the activities taking place. Some of them will make a mad dash for their food and water bowls whilst others will be feverishly going round the tank looking for places of possible escape. Resourceful crabs will be eating food that they had hidden away for later, whilst other more "fashion conscience" crabs will be trying on different shells. Quite often, your crabs will all pile on top of each other in an effort to get to the top of their tank and this will explain why their climbing frame is laying on its side in the morning. Crabs can certainly wreak havoc in numbers!
Hopefully this has covered a lot of common ground with regards to hermit crab behavior but you can find a lot more detailed information in my website.