The differences between いきなり, [急]{きゅう}に and [突然]{とつぜん} 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。 それは女の姿がその明るい電灯の光を突然遮ったためだった。(梶井基次郎「ある崖上の感情」青空文庫) It shall be noted that I think that 突然 is the most widely used suddenly-word and that it probably is misused a lot if you want to go hard-core on the definitions and origins of the words