The other is cooperation from the proposed insured.
You will have to demonstrate that the death would result in a financial loss to you, then the proposed insured would have to sign the application and complete any underwriting requirements,such as, taking a physical examination, if required by the life insurance company.
Insurable interest usually exists between spouses, siblings, relatives, business partners, and anyone with whom you may suffer a financial loss if the person died.
Here's an article that explains What is Insurable Interest.