How to update FormItem value in JSON validate response?

I am new to SmartClient and am evaluating the LGPL platform.

When using a RestDataSource against a JSON provider on a form, what is the correct way (or is there a way) to set the data of the field being validated or of other fields on the form in response to a validate request?

I also notice that the GET request being made to the server sometimes includes other fields on the parameters. Is it supposed to do this, or am I not responding to the validate correctly?

For example on a simple form with two fields, on exiting the first field it sends a validate request for just that field as expected. Then on exiting the second field it includes both the first field and the second field as params on the request.

Anyone with any ideas on this?