Is there a word for 'acting impulsively'?

Solution 1:

There are a lot of synonyms of impetuous viz - : impulsive, rash, hasty, overhasty, reckless, heedless, foolhardy, incautious, imprudent, injudicious, ill-conceived, ill-considered, unplanned, unreasoned, unthought-out, unthinking; spontaneous, impromptu, spur-of-the-moment, precipitate, precipitous, headlong, hurried, rushed.

None of them are verbs, but perhaps you can derive a verb from one of them - such as she always rushes in without thinking.

Solution 2:

"You can never tell what Harley is going to do. She always jumps in without thinking things through."

jump (in)

2.5 (of a person) make a sudden, impulsive rush to do something:
‘Gordon jumped to my defence’

Reference:
https://en.oxforddictionaries.com/definition/jump