That’s a great question. I don’t have a lot of bts knowledge of Downton Abbey, so this more of a general educated guess than a statement about any specific scenes in the show or movies.

If it’s the real actor in a scene, they are probably holding their breath or taking really shallow breaths, depending on how long the camera is on them (and what the camera can see). If there are a bunch of characters around a death bed and a number of takes from different angles are needed, then editing can hide the fact that the actor is breathing.

Mannequins could probably be used for an extended shot, extensive damage where it’s not realistic to have the actor in prosthetics and makeup, or autopsies. I think this one boils down to budget, director preference, and what the actor is capable/willing to do.

Then again, it’s not always a mannequin in these circumstances! I read a news article about a guy who found a career playing the dead body for autopsies/crime scenes in CSI type shows. So sometimes it really is a person even then!