Stephen can create a new aspect by skimming through a book or listening to a tape. If he does not already have an aspect for that subject, a new one will be created. This process does not require his attention or conscious thought.
Aspects need to be close to Stephen for him to use their information. They cannot affect the real world, so they must tell Stephen their knowledge. However, two aspects who have cell phones are able to communicate information to Stephen through them. J.C. was able to guide Stephen's hand in order to shoot people when Stephen was in danger.
Aspects are able to die. Once an aspect is dead, Stephen can no longer access that knowledge. They can die both by standard human ways to die (e.g. being stabbed) or by turning into a nightmare, which happens when Stephen loses control. Every aspect except Jenny did one of these two things while Stephen was trying to find Sandra.
At the beginning of Legion there were 47 extant Aspects; one was introduced during the case bringing the number to 48. This number excludes Aspects that died or disappeared prior to the series. (marked with a †). When Jenny was created, the number was brought up to 49, until aspects began to disappear.