The adjective fictitious began as a variant of fictional, but the words have differentiated over time. Although both can be used to mean imaginary or fabricated, fictional is often used to describe imaginative works of art and things relating to them. A science-fiction novel, for instance, is fictional, as are its characters and story. Fictitious usually means, more generally, imaginary or fabricated, without necessarily referring to fictional works of art. For example, a nonexistent illness one invents to get out of work is fictitious.
In Tiny Furniture, writer/director Lena Dunham stars as Aura, a fictional protagonist based heavily on herself. [Movie City News]
The trustee had been seeking to recover more than $1.5 million of allegedly fictitious profits from the brothers and their mother’s estate. [Washington Post]
Telepathic flight control still resides in fictional realms such as the 1982 Clint Eastwood movie FireFox. [Military & Aerospace Electronics]
The vehicle had been impounded by Douglas Police Department on 12/07/2010 for having fictitious plates. [Douglas Dispatch]