A great setting would be one that gets full sun before the trees are fully leafed out and dappled shade during the summer. They do their best in rich, organic, well-drained soil. They are perfect for that purpose. Not only will they make a great planter by themselves, they combine well with other early spring bloomers such as sweet alyssum and snapdragons. Tucking a few into a summer mixed planter will give you some early color while the summer plants are filling out.
Pansies have a very fine, fibrous root system.
When you ease them out of their pots, the outside of the root ball may be a mat of white roots. Be sure to make several shallow cuts through these roots to encourage outward growth. It is also important to plant them at the same depth they were growing. They don't do well when planted any deeper. Pansies are cool season annuals.