It would be ideal for the Action Completed checkbox to re-appear on the same entry pop-up screen in NXT, when you are creating a new action. It's inconvenient and confusing for the fundraisers to have to close the action, incomplete, and then either open the full action record or click (...) in order to complete it.
I didn't know this before but I was looking at this and I figured out that if you set the action to be a past action the checkbox is there. So if I just sent out an email to someone and wanted to add the action I had to say it was a past action and then I can enter the date of the action, check the box, and add the date it was completed too. You can't add starttime and endtime this way though.