The theme is found in the answer to 61-Across: Open Door. For the starred clues, each half of the two-word phrase or compound word answers can precede the word “door.”
16A. Comfy place = Fireside
20A. Alternative to a Crock-Pot = Dutch oven
27A. Metaphor for a sharp mind = Steel trap
36A. Gathering spot for the upwardly mobile? = Elevator door
55A. Where a cast may be found = Backstage
It happened to work out that I filled in all the second parts but had none of the first parts. One might think getting those would be easy; however, for some reason known only to my brain, it had a bit of trouble figuring them out. Overall, though, this was a relatively easy puzzle for a Thursday.