On some early diesels they had to open and close shutters, and engage and disengage cooling fans. This was based on the weather, and how hard the engine was working. One other duty of the fireman was to watch for motions before portable radios were given to train crews.
One point that was not mentioned was that a fireman was in an apprenticeship toward becoming an engineer. The engineer could give out many pointers about how to run the train, on a given line. Often the fireman ran under the engineer’s supervision. This is how the craft used to be taught.