Note to Twelfth Night, 1.2.41-44, "O that I served that lady / And might not be delivered to the world, / Till I had made mine own occasion mellow, / What my estate is!"

Viola wishes that she could be a servant to Lady Olivia until she can make the time ripe ("mellow") to reveal that she is not really a servant, but a lady. Why she wishes to go into disguise is not clear. As a lone woman in a strange country Viola would naturally be exposed to many dangers (though she doesn't talk about them), so probably we're to assume she has her reasons.