There are 2 simple way to do that.
1. Create a visible button which has a script to Go To Related Record (using the layout to the customer detail). Single Step is fine.
2. Create an invisible button, which just same as above, but you have to arrange this button on the top of the field or merge text on the portal AND , DO NOT make the button size over or crossed the portal. The script is same as above.
Single Step Script : Go to related records (using the customer detail)
None of your suggestions match the request.
The request is to display this detail on the current layout.
Unfortunately that won't work as I want to stay on the same layout. It's a summary page of sorts which has a list of all customers in one section and an information area in another part of the layout. I want to click the customer in the portal and the information area shows the customer. Image shown here gives an idea. I want to click the button by Jim Smith and the details section shows his details
One way: add a global gSelectedCustomerID field.
In the relationship graph, duplicate the Customer table occurrence and call it Selected_customer.
Create a relationship from gSelectedCustomerID to the CustomerID of this table occurrence.
Add fields from this table occurrence to your layout.
Your script to select the customer would put the selected customer ID into this global field.
If you need to have details displayed indefinitely I would use Bruce's suggestion—I use that method extensively and it works well. If you only need to see the details temporarily then another technique is to use a popover button on the portal row which you set to display further detail of that portal record. This method has the advantage that only the button itself is taking up screen space, but the popover can be opened up whenever you need to refer to it.
"If you need to have details displayed indefinitely I would use Bruce's suggestion..."
And note that in message 5, Bev links to an excellent example that uses this technique.
Kudos to Bev for finding that, seems like a perfect example for this post.