No, if the texture of your hair is already straight, there is no need to use a flat iron. If that is the case, you may need to use a curling iron with a large barrel to add some body.

Look for shampoos and styling products that contain "panthenol" (usually in "thickening" products) It will open the cuticle layer of the hair and make each strand appear thicker. If those products cause "flyaways" use a LITTLE anti frizz serum (it will not reverse the effects of the thickening agents, rather just hold down the flyaways). Then if you want the "separated" look, try curling with a large barrell and using spray wax.