One of the benefits of playing a regular character is the familiarity with the role. Most actors get quickly acquainted with the characters they play, whether in the theatre or in a film or TV show, you develop ways of getting to know them. Some actors write out histories, some assume the personality of their part and stay 'in character' for the whole job. But on stage and in film, the character is a collaboration. The playwright's words are sacrosanct in the theatre, not a syllable can be changed without the author's permission and the actor's job is to work with the director to bring the story to life.
On film, famously, writers can be treated very badly, and the script is a little more malleable. However, the character is still a collaboration between the writer, director and actor. Other departments are also influential, the costume, make-up and set designers all make decisions about the characters in a film. Most importantly, once the film is finished, the director and editor get together and make a completely new person out of the one you pieced together over the months of shooting. At this point, the music may be making more performance decisions than you did.
Of course, all these things happen on television shows too. But something else comes into the mix. After a while in a role, the actor starts to know the character better than anyone else ever could. They spend all their working life with them and they have opinions on every choice the character makes. Meanwhile, directors, writers, even series creators, can come and go. Writing teams have a chance to react to what they see on the screen. One character responds unexpectedly to a scene. The writers might want to explore why. A couple of other characters are exciting when they are on screen together, the writers send them out on a date and see what happens. As an actor, this can be both a blessing and a curse. One week you may be celebrating your challenging new storylines, another might find you indulging your paranoia as you search in vain for any non-expositional dialogue.
For the last month I have been visiting the wards on 'Holby City' but the longest time I have spent with a character was on 'Heartbeat'. For two years, I charged about the North Yorkshire moors in my 1960's Citroen Safari, happily dispensing medicine to the denizens of Aidensfield. I learned a lot about the stamina needed to keep up with the schedule and quite a bit about tenant farming in modern Britain. Now there is a tough job.