I agree with @Nanren888 that the speaker believes that Mr James did not actually accomplish anything. In fact, the speaker is not curious at all: they know that there will be no useful or true information in Mr James's reply. They are asking this question to put Mr James 'on the spot' and perhaps to force him to admit how little he accomplished. My guess is that Mr James had been showing off about his supposed accomplishments and the speaker is trying to prove that he is either lying or exaggerating. That said, I do think that the meaning of 'just' is 'exactly' in this context. Perhaps Mr James had been boasting in very general terms about how much he had done. The speaker is now saying 'OK, then. Tell me exactly what you did', knowing that Mr James would be unable to specify. The speaker is asking for 'details', precisely because he knows that there are no details to be given (or that Mr James will be exposed as a liar if he tries to provide any).

Exactly? No. The structure is suggesting that he did not accomplish anything, or at least not what he wanted.