Think of some great sitcoms. Fawlty Towers. Absolutely Fabulous. Peep Show. All based around great characters: Basil Fawlty, Patsy and Edina, Mark and Jeremy.
Now think of Basil without Manuel, Patsy and Edina without Saffron, Mark and Jeremy without Super Hans. What would happen? Those sitcoms would lose more than just a secondary character, they'd lose a vital part of what makes them special.
The right B character can make a sitcom a classic.
B characters aren't as complex as the main characters, in fact they're usually stereotypes, but they play a vital role in the way they interact with the main characters.
Why are B characters so important?
When the B characters show up in a sitcom something changes. B characters always behave the same way - but they change the way the main characters behave.
How do B characters change the behaviour of the main characters?
The B characters are often like cartoon versions of the A characters. They're like an exaggerated offspring of one quality in a main character. They can represent exactly what the A characters don't like - about themselves. They reflect them in a distorting mirror.
In 'Will and Grace' Jack is exactly the kind of flamboyantly camp gay man that Will would dread to be seen as. Yet Will knows there's a side of him that could be like that. And Karen is the type of crazy, raddled New York fashionista that Grace suspects she could become if she just let things slip a bit and let one aspect of herself out of the cage.
Sid James was the perfect side-kick for Tony Hancock, the lower-middle class snob, because he showed Hancock everything he was trying not to be, and often lured him into betraying himself - or taking such pains not to betray himself that he became ludicrous.
How to create a B character.
Pinpoint the quality that the A character most hates about themselves. Create a B character who embodies this quality. Write some dialogue in which the B character offers advice - like the A character's bad angel, luring them to betray themselves or to react against the B character's attitude so strongly that comic tension or conflict is created.
Now you've got the makings of a B character - and now you can start to play, because:
B Characters are a lot of fun!