You must still pay all costs and follow any timing restrictions and permissions for creature spells you cast from exile this way. Normally, you'll be able to cast them only during your main phase while the stack is empty.

Dawnhand Dissident must be under your control in order for you to start casting a creature spell using its last ability, but it doesn't need to stay under your control throughout that process or be under your control as the spell resolves. If, for example, you sacrifice Dawnhand Dissident to pay an additional cost of the spell or it's destroyed in response, the spell will still resolve normally.