Ainley was the fourth actor to play the Master and the first to portray the Master as a recurring role after the death of Roger Delgado in 1973.
Ainley's great love of the role is often mentioned in DVD documentaries and commentaries. Eric Saward claimed that Ainley used to introduce himself over the phone by saying "This is the Master" and then laughing.
Ainley didn't do much other acting. Colin Baker said "He only ever wanted to play the Master".
Ainley remained unmarried throughout his life. Not to imply anything here, but his Master was pretty damn camp.