Locomotive Steam Characteristics

I have heard that each individual steam locomotive has its own steaming habits and patterns. Each locomotive in an identical batch built at the same time could fire differently, i.e. the K-28s or K-36s.
What contributes to this phenomenon? What makes a locomotive a "good" steamer as opposed to a "poor" or "bad" steamer?
Can a locomotive steam well as a wood burner and steam poorly after being converted to oil or coal?(and all the other possible changes of one type of fuel to another)
Can an overhaul change steaming characteristics of a locomotive, ie from bad to good and vice versa?
Thanks.