This comes via Facebook from a priest reader:
What can you tell me about the use of the amice over the surplice for deacon chaplains and Assisting Priests? The AP at my first Mass wore the amice that way.
There isn’t really too much to say.
There are a few occasions wherein there may be an assistant priest, such as when a bishop pontificates or for the first three solemn Masses of a newly ordained priest. The assistant does certain things with, for example, the book and especially in the case of the newly ordained, makes sure he doesn’t make a grave mistake.
In this screen capture you can see an assistant priest to the left, in the cope.
Again… the AP is on the left, though he has his cope properly closed over his knees so that you cannot see the cassock. The deacon "chaplains" sitting with the Cardinal in this shot have their surplices.
He dresses in his cassock, with a surplice (or rochette if can wear one). He put on the amice over the surplice and then the cope. I think the idea is that it is he not quite a sacred minister in the sense that the celebrant and deacon and subdeacon are. He is sort of in between being an MC and a sacred minister. The amice? Dunno… that’s just the way it is.
You might be interested to know that in olden times, in processions, priests and deacons used to wear the vestment of their rank, chasuble or dalmatic, but over the surplice, not over an alb, also with the amice over the surplice.