Stephano and Trinculo may be nasty to Caliban, but if I were Caliban I would probably still put my trust in Stephano.
I know from past experience that Prospero is not going to be good to me, right? In that case, it would seem like it would be better to take my chances with someone who might treat me decently. After all, Stephano does seem to be willing to take my side.
So if I were Caliban in this scene I think I'd do pretty much what he did. There's not much that could really go worse for me so I might as well give it a try.
I am not entirely sure to what particular part of this scene you are referring to. This scene is part of the comic sub-plot with Caliban, Trinculo and Stephano, when Caliban tries to encourage Stephano to challenge Prospero, steal his books and take over the power of the island, and Miranda with it. However, unbeknownst to them all, Ariel is eavesdropping in on their conversation, being invisible, and takes part in the action, causing great hilarity and confusion.
However, one less amusing aspect of this scene is the way in which Stephano and Trinculo treat and regard Caliban, which is of course completely unacceptable by today's standards. Stephano calls him "Servant monster" and Trinculo refers to him as a "deboshed fish." Of course, Caliban does appeal to Stephano to stop such abuse, but it is highly ironic that he is so quick to exchange the lordship of Prospero for such an unworthy replacement. It is easy for us to look into this scene with our 21st century perspective and condemn what we see, but at the same time we must remember that circus freaks are popular even in today's world. If I were Caliban I would of course be outraged at the way in which I was being treated, but at the same time we need to remember that Shakespeare may be forcing us as an audience to examine our own attitudes by encouraging us to laugh at Caliban, his ignorance and the way he is treated.
thnx for da help guyz