You can only become an agent under a power of attorney if the principal names you in the document. Power-of-attorney forms and procedures differ among states, but all require that the principal name the agent and describe the scope of authority. Most powers of attorney concern the principal's financial affairs or medical care. In some states, medical powers of attorney are termed proxies or healthcare directives. A durable power of attorney remains effective after the principal becomes incapacitated.
If the principal is adult and mentally competent, he is free to make a power of attorney and select his own agent. Despite the term power of attorney, the agent selected need not be an attorney nor have legal training. The American Bar Association suggests that the best agents are those persons in whom the principal has the most confidence since the authority granted by a power of attorney can be subject to abuse. The spouse of a principal has no say in whom he names as his agent.
A power of attorney allows someone else to step in and act for the principal when he cannot or chooses not to act himself. A financial power of attorney can be general, giving someone the right to make any decision or take any step the principal might himself make regarding his affairs. Alternatively, the scope can be very limited, such as a power of attorney to allow an agent to sell certain shares of stock. Note that one spouse's financial power of attorney does not give his agent the authority to make decisions about the other spouse's property. Most health-care powers of attorney are durable and intended to name an agent to make medical decisions when the principal is incompetent.
If the principal is legally competent, he is free to revoke a power of attorney at any time and for any reason. While a spouse can pressure her husband to execute a revocation document, she has no legal authority to challenge his choices. On the other hand, if the principal is incompetent, a spouse can challenge the actions of an agent and request a court to revoke the power of attorney. A court only acts if the agent is abusing his authority or failing to act in the best interests of the principal.