Fill form after mount in v3?

I have a form next to a table, and the form contents change based on what record the user clicks in the table. In v2, I was able to call

$this->form->fill()

$this->form->fill()

from anywhere in the component, but in v3 it only seems to work if I call it from

mount()

mount()

. Is there a different way I should be doing this now?