When it is sudden: unexpected and fast. Just moving hands from chin to knees wouldn't be too sudden unless she practically slapped her knees. Don't just indicate an action that happened "sourcelessly" from inactive environment. Indicate a fast, rapid and unexpected action. "Suddenly she drew a gun and put it to his head."
"All of sudden" is more of a spoken expression. You can have the characters use it in dialogue. For actual narration, "suddenly" is preferred.
EDIT: Following Lauren's comment. It's not forbidden, it's not a sin, it's just a common crutch, and easy to abuse, cheap. If you're about to use it, think if there's a better way to say it. Not always, but quite often, there is. Also, remember: It's an adverb.