Concerning The Great Gatsby, the primary character guilty of unrealistic, idealistic thinking, and behavior that stems from that thinking, is Jay Gatsby. And his thinking is the catalyst for the action and the reason for the novel.
Gatsby has an unrealistic and idealistic view of Daisy and of his relationship with Daisy. The consequences are that he lives five years of his life with the sole purpose of winning Daisy back and recreating the past--his past relationship with Daisy. Everything that occurs in the novel--the accident that claims a life, Gatsby's murder, etc., results from Gatsby's illusions.
The past he wants to recreate is an illusion. Daisy never loved him like he loved her. She was not longingly looking back to her relationship with Gatsby and wishing she were with him instead of with Tom. Gatsby is deluded. He thinks they had something poignant and special. But Daisy does not. She could love him, and possibly does in her own way when she talks of leaving Tom for him, but she won't dismiss the last five years of her life and marriage and announce that she never loved Tom, as Gatsby insists she do. This shows that Gatsby's view of their relationship is an illusion, and demonstrates that the consequences suffered in the novel are for nothing.