How did minor characters make a major impact on the play?

Maria is Olivia's gentlewoman-maid, a witty, wily woman who has some affection for Sir Toby. She often points out what others fail to mention like the drunken intoxication of characters like the "foolish Knight." She also provides much witty comedy and commentary about life in her exchanges with Feste the clown. 

Fabian is a member of Olivia's household with responsibilities that are never explained. He is a sort of mid-play replacement for Feste, taking part in the plots against Malvolio with Sir Toby and Sir Andrew. He doesn't seem to have much of a personality, but is included in the play for the purpose of exposition‹and he is also good at stating the obvious for the audience.