Question:

How to query the Approval Action history for a Salesforce approval process?

Answer:

The user can run a query as shown below (Note: Replace the Event ID in the where clause): 

SELECT Id, Status, (SELECT Id, StepStatus, Actor.Name, Comments 
FROM StepsAndWorkitems ORDER BY CreatedDate ASC) 
FROM ProcessInstance WHERE TargetObjectId = 'Event ID'

Related Documentation:

CRM Help Documentation: Set Up an Approval Process