They are only 'casual' acquaintances. [=they know each other but not very well]
She takes a 'casual' [='nonchalant'] approach to her homework.
He was dressed 'casually' in jeans and a sweatshirt.
(..) on the Iron Throne. Join us. Together, we can end her tyranny." Sounds like a charmer. Of course, the casual mention of a Dothraki horde, a legion of Unsullied, and three dragons... a bit less charming. What? (..)2017 TvShow: Game of Thrones Title: Stormborn Season: 7 Episode 2