Fictional and fictitious are both adjectives that mean roughly "made up" or "invented." The difference between the two is how they are typically used rather than what they mean.
Fictional is usually used to describe something in literature such as a fictional character or a fictional story.
If you will practice being fictional for a while, you will understand that fictional characters are sometimes more real than people with bodies and heartbeats. — Richard Bach, American author of Jonathan Livingston Seagull
Fictitious is usually used to describe a lie or invention that happens in real life.
Sane and intelligent human beings ... carefully and cautiously and diligently conceal their private real opinions from the world and give out fictitious ones in their stead for general consumption. — Mark Twain in Mark Twain in Eruption: Hitherto Unpublished Pages About Men and Events
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