Expression is tied up with fixation. Copyright subsists in a work that is fixed in a tangible medium of expression from which they can be reproduced, perceived, or communicated. 17 USC 102
This expression is protected, not the idea. However, copyright infringement can occur even when there is not an exact copy. First, courts use a substantial similarity standard to determine if infringement has occurred. Second, when a character or plot is sufficiently developed, taking that character or that plot can be infringement, even if not expressed in the exact same manner.
"We do not doubt that two
plays may correspond in plot closely enough for infringement [...] the less developed the characters, the less they can be copyrighted;
that is the penalty an author must bear for marking them too indistinctly." Nichols v. Universal Pictures Corporation, 45 F.2d 119 (2d Cir. 1930)
Some characters or plot elements are so common to a genre that they are either not considered "original" enough to get copyright protection, or can be taken by others without being considered infringement. This is the scènes à faire doctrine.
"Stock scenes and hackneyed character types that "naturally flow from a common theme"—are considered "ideas," and therefore are not copyrightable. But as plots become more intricately detailed and characters become more idiosyncratic, they at some point cross the line into "expression" and are protected by copyright." Suntrust Bank v. Houghton Mifflin Co., 268 F. 3d 1257 - Court of Appeals, 11th Circuit 2001
See Amanda Schreyer's An Overview of Legal Protection for Fictional
Characters: Balancing Public and Private Interests for many more cases and examples of the idea-expression dichotomy in action with respect to fictional characters.