Time: surprise
急に
急に means that something happened unexpectedly or quickly.
Example
急に電車が現れた。 A train suddenly appeared.
ふと
This means “suddenly”, “unexpectedly”, “unintentionally”. Emphasis is on the lack of intention.
Example
ふと通りに1000円を見つけた。 I found 1000 yen on the street by chance.
たとたん
Means “just as”. Used for an action that happens just at the moment something else occurs. Can convey a nuance of something unexpected or surprising. Used with verbs in the past form (た).
Example
部屋を掃除したとたん、うちの猫が吐いてしまった。 As soon as I cleaned my room, my cat vomited.