Are you usually casted if you have a broken foot?

Depends: It will depend on what part of your foot you broke. The xray will determine that. If the toes need to be immobilized during the healing process, a cast may be used.

Foot cast or splint: Yes, the approprate treatment for a foot fracture is to put it in a cast or splint to immobilize the fracture after it has been reduced (xrays very helpful). Partial or complete non-weight bearing with crutches is indicated depending on the fracture for the same reason: gravity when one loads the foot can disrupt the fracture reduction/ alignment and cause it to be displaced or mal-aligned.

Sometimes: That is any option depending on the type an location of the fracture.