The hand of fate is evident in Twelfth Night in a few different places; however, for the most part, consequences and resolutions happen as a result of characters' choices.
The two most obvious ways in which fate is evident in the play is with respect to the fact that both Viola and Sebastian managed to be rescued from the shipwreck. Viola was saved by a sea captain and so was Sebastian saved by a different captain of a different vessel. Viola's captain even comments on the role chance, or fate, had in rescuing Viola and says there is the same chance that Sebastian was rescued as well, as we see in his line, "It is perchance that you yourself were saved," meaning, "It was by chance" that Viola was saved; therefore, there is a possibility her brother could have been saved as well (I.ii.7).
A third way in which fate has a hand in the action is with respect to the fact that both Viola and Sebastian wind up in Illyria. Being in the same place at the same time further provides the opportunity for them to be mistaken for each other, which also offers the opportunity for Olivia to marry a man rather than continue to be in love with a woman posing as a man. Hence we see that fate plays a role in coupling Olivia with Sebastian plus even reuniting the brother and sister pair.
However, while Olivia's marriage to Sebastian has a great deal to do with chance, Orsino made the choice to ask Viola to be his wife, showing us that the play's resolutions also depend on choices and not just on fate. In addition, Malvolio was persecuted by the other characters due to the characters' own choices as well as Malvolio's own choices. Hence, the play has even more to do with choices than it does with fate.
A profound sense of Fate underlines all Shakespeare's tragedies;but the characters of his pure comedies "are so confident in there happiness that they can play with it,and mock it,and put it to trials that would break fragility.They are equal to circumstance,and the most surprising adventures do not disconcert nor depress them.In a sense they too,like the tragic heroes and heroines,are the antagonists of Fate.But Fate,in the realm of comedy,appears in the milder and more capricious character of Fortune,whose wheel turns and turns again and vindicates the merry heart.'Who can controle his Fate,'says Othello."tis but Fortune;all is Fortune",says Malvolio,when he believes himself to stand in favour with Olivia;'Jove,not I',is the doer of this,and he is to be thanked'. Olivia,ensnared by thebeauty of the disguised Viola,gives voice to the same creed:
I do not know what,and fear to find
Mine eye too great a flatterer for my mind;
Fate show thy force;ourselves we do not owe
What is decreed,must be;and be this so;
And Viola,in like fashion,trusts to the evet-
Time,thou must untangle this,nor I;
It is too hard a knot for me to untie.