Give the central character relationships that bring out different aspects of his personality. For example Fawlty is subservient with Sybil but a cruel tyrant with Manuel. Blackadder is a crawler with the Queen but a sarcastic bully to Baldrick. The simplest way to do this is to put the characters in some form of hierarchy.
It's not just a question of having a consistent character - but having a blend of characters who bring out different aspects of each others' personality. It sounds complicated, and it is a bit. But if it's not in your sitcom at the start, it'll be very hard to work in later. Wise words.