word choice - The differences between いきなり, 急に and 突然 - Japanese . . . 突然 is more formal, seen in writings often but still fine for colloquial use Same as いきなり in meaning 唐突 is the same as 突然 but used with human behavior, not inanimate objects, trains, astroids, etc Example: 唐突なカミングアウト (unexpected and sudden coming out of a person about something)
Nuances of: ふっと、ふと、ひょっこり、 どっと、 さっさ、不意に、唐突、突然、急に and いきなり。 On the other hand, 突然 is used for really surprising things like ''突然に it started raining men'' You cannot interchange those two words in these 3 examples Some easy examples are given on the nlpt preparation site 授業中に、急に眠くなりました。