I say potentially because I usually let the captain set the field, but after last season even though it was pretty awful, I've got some sense of the kind of field I'd set at the start of the game till I settled and had some sense of how my bowing was going and how the bats were dealing with my bowling.
The diagram at the bottom indicates my bowling line - over the wicket at middle and off - looking to turn the ball away from the edge of the bat. The bloke that's in the 'Square 3rd man position' (11) would be brought up to a position at Gully depending on how the batman was playing the ball, if say he was playing straight bat shots trying to hit it through the covers with a chance of inviting the edge. If I was getting on top of him and he was playing defensively he may then be brought into short extra cover or silly mid-off, with point being pushed back slightly. In the same way Deep Square Leg would be brought up into a more conventional position. (10) at fine leg would stay in that position for Wrong Uns. Whether that makes any sense or not to other bowlers I don't know. A lot of what you do is dictated by the batsmen and how they're playing and what their strengths are. Reading the batsmen and responding to what they do is a whole new facet of the game that takes a great deal of nous. At the moment it's been interesting to see Ricky Ponting in the Ashes moving blokes around in response to where the balls being hit in the same over and he's getting slated by the commentators, so it's not easy.