Opposite words for suddenly:
pokily, tardily, sluggishly, slowly, hesitantly, gradually, obviously, leisurely, laggardly.
Usage samples:
  1. Suddenly Mary became very, very ill.
    "White Queen of the Cannibals: The Story of Mary Slessor", A. J. Bueltmann.
  2. Every thing about her was suddenly silent,- not a sound to be heard.
    "Rico And Wiseli Rico And Stineli, And How Wiseli Was Provided For", Johanna Spyri.
  3. Suddenly he looked around.
    "The Impossibles", Gordon Randall Garrett Laurence Mark Janifer.
  4. Quite suddenly, they were brother and sister together.
    "A Bride from the Bush", E. W. (Ernest William) Hornung.