The same person can take on the role of executor and take on the role of successor trustee; however, the roles are different.
The executor takes care of the body, make funeral arrangements, goes through the house, checks what to do with all of the tangible personal property and deals with the more personal emotional side of someone's death.
The trustee deals with all the financial assets, selling things, distributing things, collecting the IRA beneficiary designations and all the sorts of things that are required to bring the assets under one umbrella so they can be distributed.