Elwin Ransom
Elwin Ransom may be based on C. S. Lewis' friend J. R. R. Tolkien ("Elwin" means "Elf friend" in Anglo-Saxon), though he seems to have autobiographical elements. In "That Hideous Strength" Ransom, with his royal charisma and matter-of-fact breezy acceptance of the supernatural, appears less like Tolkien than like Charles Williams (or some of the heroes in Williams' books).
Mount Doom
Reviewer Bart Higgins noted the similarity between Tolkien's Mount Doom and the volcanic caverns under Venus depicted in "Perelandra" by Tolkien's friend C.S. Lewis. "In both cases, the book reaches a cataclysmic scene of a struggle in an underground cavern, ending with an evil being falling into volcanic fire and being consumed. Since it is well known that Tolkien and Lewis frequently shared with each other their respective works in progress, it is reasonable to assume that the resemblance is no coincidence".