Correct way (if you are not trying to reset the value of the hidden_field input) is:
f.hidden_field :method, :value => value_of_the_hidden_field_as_it_comes_through_in_your_form
Where :method
is the method that when called on the object results in the value you want
So following the example above:
= simple_form_for @movie do |f|
= f.hidden :title, "some value"
= f.button :submit
The code used in the example will reset the value (:title) of @movie being passed by the form. If you need to access the value (:title) of a movie, instead of resetting it, do this:
= simple_form_for @movie do |f|
= f.hidden :title, :value => params[:movie][:title]
= f.button :submit
Again only use my answer is you do not want to reset the value submitted by the user.
I hope this makes sense.