You cannot take out a life insurance policy on someone else's life without his or her knowledge and consent. Usually, the person needs to sign the application for coverage.
You must have an insurable interest in the person you want to insure for life insurance which means you either rely on the person for financial support, or you stand to lose out financially if the person were to die.
Insurable interest may exist between spouses, family members, and business partners, among others.