Official comment
- Add the fields to the Smart Form
- Click the "Conditions" button

- Create a Rule that hides the field you later want to conditionally reveal

- Add another Rule that reveals that field based on the entry in another field. In this case, if the drop down box for the field "Appointment Time" is 10:00 AM, then I show the Company field:
- Make sure you have the two Rules in the right order. You must first hide the field based on it being empty, then show the field once your condition is met:

Here's a video clip of it in action: http://screencast.com/t/Dc8huhxzGA
When you select 10 AM in the Appointment Time drop down, the Company field appears.
Comment actions