Hello.
In my case, it is proceeded when jira issue is created.
For example
Jira issue Field
1#Custom_Field : User Picker (single user) << Field Required
2#Custom_Field : User Picker (single user)
Q1.
If the 2#Custom_Field value is included, Change Assignee(2#Custom_Field) and
Go to Jira's specific status
Q2.
If the 2#Custom_Field value is Not included, Change Assignee(1#Custom_Field)
I don't know how to implement it using Script-Runner.
Can I help you?