How do I delegate a task to a different assignee?

Sometimes it may be necessary for a different person to perform a task. For example, when an assignee goes on holiday. The best workflows have exceptions built into the design. So for example, you may want to assign a step to two users, one as a back-up, and set the assignee policy to require only one. Another approach which gives even more flexibility is to assign the step to a special role. Then you can swap users in the role when ever you like.