A separate layout or a separate layout that is displayed in a separate window? Both are possible.
Sticking with just the separate layout option for now (the new window options is just a slight variation of the same approach):
You can either script a find or use Go To Related Recors to switch to a detail view of your selected record in the list or portal. GTRR is particularly useful for when you select from a portal as you can use the same relationship that defines the portal in your GTRR step to jump to the detail view layout.
Using a scripted find, your script would look like this:
Set Variable [$ID ; Yourtable::primarykeyfield] //any field that uniquely identifies your record will work here
Go To Layout [//specify detail layout]
Set Field [YourTable::primarykeyfield ; $ID ]
With a portal, you can even do this all on one layout. You can have a selection portal in part of the layout and use the rest of the layout to display the details of the selected record.