To officially serve as an executor, you must be formally appointed by the court at the initial probate proceeding. Until then, you are simply a nominee with no legal authority over the estate.
In most cases, a will creator informs their chosen executor before their passing, allowing the executor to prepare for their role. This way, when the will creator dies, the named executor will know how to locate and access the will and can begin the probate process by lodging the will with the court.
However, if no executor was named in the will, if the named executor declines their appointment and no alternate is listed, or if the court refuses to appoint the named executor, the court may appoint an administrator to oversee the estate instead.