develop friendship with co-star

Because it just specifies co-star, when you invite over all your co-workers, only one of them will advance the prep tasks. And sometimes they might take a while to show up . You just have to talk to all of them until you find the one that is your specific co-star because sadly the game won't tell you. When you talk to them just you can just use either Get to Know or Ask About Hobbies and Skills,  both these will advance the task.