Anders Bengtsson has a post up showing how to alter the default activity execution account when you want to use a different account than the default one. He steps you through and shows you what it looks like.
By default all activities are executed with the account you have specified on the Orchestrator Runbook Service on your Runbook servers. In some scenario you need to configure a activity with a specific account, you can do that on the security tab on each activity.
Read the full post: Run a runbook with a specific account