How to configure fields as 'required' dependent on other field values in Business Process Flow

In my hypothetical process flow below... in stage 1 I have an option set 'Sales type'  with options 'Email' and 'Phone'. On the next stage I want to show only particular steps based on the option set in stage 1. For example:

Stage 2 steps if 'Email' selected:
Meeting: Yes / No
Call: Yes / No

Stage 2 steps if 'Phone' selected:
Meeting: Yes / No
Call: Yes / No
Proposal: Yes / No

We also want to make 'Proposal' mandatory (required) to progress past stage 2.

What I have tried so far:

• Setting a business rule on the form:

If 'Email' Selected - hide 'proposal' - set to not required  - if 'phone' selected show 'proposal'

Setting 'Proposal' as required on the form

This does not work as progression an opportunity for email still shows incomplete steps.