I could be wrong (usually am, but) this is a common case of what is known as a character actor.
There are certain actors, Bill Nighy definately being one, who always play a certain role. They are exceptionally good at it, but it is difficult to see because they always seem to play the same character. Producers and Directors love them because they always put in a sparkling performance, help to lift a film / TV program, and bring in the viewers / punters. Their agent loves them because it is easy to get them parts - if you want a certain sort of character, their name is always at the top of the list. Which is why we see these actors in so many films or shows. The actor themselves, although they would love to play different characters, accept that their best career choice is to accept their character role, and just concentrate on playing it as well as they possibly can.
However they themselves are not the character they play. And if you ever see a poor character actor, you will notice the gulf in class between the two. And if you are even luckier to see them in something where they are not playing their character, e.g. a theatre production, then you will be blown away by how they seem to be someone completely different.
Some actors, thinking in particular of Hugh Grant, Hugh Laurie and David Jason in particular, manage to play a great variety of roles and excel in doing so. But this takes a lot of effort and dedication, and presumably a good chunk of luck too.