Post functions allow you to perform specific actions upon the transition. Following are some useful examples.

import com.atlassian.jira.component.ComponentAccessor
 
def commentManager = ComponentAccessor.getCommentManager()
def currentUser = ComponentAccessor.getJiraAuthenticationContext().getLoggedInUser()
    commentManager.create(
        issue,
        currentUser,
        "This is my comment",
        false) 

import com.atlassian.jira.component.ComponentAccessor
import com.onresolve.scriptrunner.runner.util.UserMessageUtil
 
def versionManager = ComponentAccessor.getVersionManager()
def projectManager = ComponentAccessor.getProjectManager()
def project = projectManager.getProjectObjByKey(issue.projectObject.key)
def versions = versionManager.getVersions(project)
def firstUnreleasedFound = versions.find{! it.released}
 
if (issue.fixVersions.size() == 0){
    if (firstUnreleasedFound) {    
		issue.setFixVersions([firstUnreleasedFound])
		issue.store()
	} else {
    	UserMessageUtil.error("MANUAL INTERVENTION REQUIRED!!! No Unreleased Fix Version Found. Please add a Fix Version manually to this ticket....")
	}    
}

import com.atlassian.jira.issue.CustomFieldManager
import com.atlassian.jira.issue.fields.CustomField
import com.atlassian.jira.component.ComponentAccessor
 
CustomFieldManager customFieldManager = ComponentAccessor.getCustomFieldManager()
CustomField cfUID = customFieldManager.getCustomFieldObjectByName("User ID")
def cfUIDvalue = issue.getCustomFieldValue(cfUID)
 
issue.reporterId=cfUIDvalue

If you're using this code on issue creation, make sure the script is after the “Create the issue originally” task.

import com.atlassian.jira.component.ComponentAccessor
def watcherManager = ComponentAccessor.getWatcherManager()
def userManager = ComponentAccessor.getUserManager()
def watchUsers = {usernames ->
    usernames.each {
        def user = userManager.getUserByKey(it.toString())
        if (user) {
            watcherManager.startWatching(user, issue)
        }
 
    }
}
def users = ["abc", "def", "ghi"]
if (issue.projectObject.key == 'KEY') {
    watchUsers(users)
}