In The Bronze Bow, what plan do Joel and Thacia devise so that Joel can get the information he needs without being suspected?

I take it that you are referring to Chapter Sixteen of
this excellent novel, when Daniel, Joel and Thacia meet to discuss how they can gain the
names of the rich who are going to be away from home at a banquet for
Rosh.

Note how this chapter reflects Joel's eagerness to
work for Rosh and to do anything that will oppose Roman rule. Thacia's idea is that her
brother sells fish to slaves of these rich families, who might reveal what their masters
are doing. Thacia then says she will dress up as Joel and go into the city, providing an
alibi for Joel whilst he engages in this plan to gain the information Rosh wants.
However, Thacia is reluctant to pursue this plan, as she thinks that Jesus would be
disappointed in her to discover her involvement in
deception:

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"It's just that I don't want to face Jesus with a
lie. I couldn't bear the way his eyes would look at
me."

Thus it is that Thacia
and Joel plan to fulfil the request of Josh and remove Joel from being suspected by
Thacia herself pretending to be Joel, thus giving him an alibi.