The characters in ‘Doctor Who’ are quite stereotypical in a way. Also the absence of some characters in some episodes (such as Barbara’s absence in some episodes e.g. an episode of ‘The Sensorites’, and The Doctor’s absence in some episodes e.g. an episode of ‘The Keys of Marinus’) is explained by Barbara being up on a ship above a planet and The Doctor being in a different location. This is so the actors and actresses could have a holiday break.
The content in an episode can be closed or open and it can be either single-strand or multi-strand. I think single-strand means a single story and multi-strand means a story arc.
The presence of characters like The Doctor changes things as people get happy and are excited to see him as they know The Doctor is here to save the day and they know he will save it at any cost. It’s not just the characters on screen (his companions and his friends) that are excited and happy to see him, but also the audience watching the TV at home.