Since Casey is killed within the first ten minutes of the film, we don't get to see Casey's interaction with other characters. I don't count her interaction with Ghostface (Billy and Stu), because she didn't know who her killer was until she was dying. Casey had important relationships, however, that led to her death.

Sidney Prescott

Sidney First, let's look at Sidney. Sidney is the hero and main character of Scream as well as Billy and Stu's real life movie they were creating, discussed in their final act.

Sure it is, Sid. It's all a movie. It's all one great big movie.

Casey sat beside Sidney in English. I don't think they interacted much, or were even friends. Sidney just knew who she was. I think that made Casey a perfect initial victim (aside from Maureen Prescott, of course) for Billy and Stu's plan. Knowing Casey was enough to disturb and scare Sidney when she found out about her death, and I think anything else would have been too much. Billy and Stu played a cat and mouse game. It wasn't their intention to go straight after Sidney. They needed to scare her, make her ready, and deliver the full impact of it all in an incredibly epic way. Killing one of her close friends right away would have done a lot more than disturb and scare her, and they wanted to play with Sidney. Casey was just a bleep on Sidney's radar... someone she would notice, and I think Casey was chosen partly for that reason.

Stu Macher

Stu Stu is another big character we should discuss. There's not a lot known about Casey's previous relationship with Stu, but with what we do know, it's easy to connect the dots that made Casey a victim. Casey and Stu used to date. Randy states in one scene that Casey broke up with Stu for Steve, a popular jock. Stu then clarifies that he dumped Casey for Tatem, Sidney's best friend. Since Casey and Steve were victims of Stu, I think it's safe to assume that Casey was the one who broke up with Stu. Butthurt, perhaps, Stu is probably the one who selected Casey as the victim for Billy and him to practice on before going after Sidney.