I think we might first need to differentiate between a formal hearing and a formal nullity trial. They’re two different things.
If a declaration of nullity is sought, the exact procedure to follow depends on the grounds. If it’s something you can prove “on paper” (for example, one party was already married, they were too young, there were no witnesses, they married outside the Church without a dispensation, they were too closely related, etc.) no formal nullity is required. These cases are fairly simple to prove “on paper” so my expectation is that you would not even need to come in for an interview.
If the grounds are more nebulous (e.g. things like insufficient understanding of the nature of marriage, lack of valid consent, decision not to be open to having children, etc.) a formal nullity trial is required. This involves the petitioner (sounds like this is you) coming in for an interview with the parish priest or a tribunal official. Your former spouse will also be contacted; whether or not they participate is their choice and the process can proceed without them if they choose not to be involved. Next, your witnesses will be interviewed. The interviews are conducted separately and done either by the parish priest or a tribunal official at a time convenient to the interviewee. (I think sometimes questionnaires are sent out instead - they do interviews in our area.) Once this is done, the tribunal will indicate whether additional evidence is required - this could include things like police reports, counselling records, medical documents, etc. When all of the evidence is in, the tribunal will issue a decree and you’re permitted to come in and examine the evidence if you want to (you don’t have to). Your ex spouse is also given this right. At this point, you can choose to disagree with any evidence or you can add more evidence if you want, but this can delay proceedings. Once all of this is done, the tribunal will reach a judgement as to whether the marriage was valid or not. If the decision is affirmative (i.e. marriage not valid), they will submit the case automatically to an appeals tribunal who will go over the evidence again. (You should not hear from the appeals tribunal unless there’s a problem or they want to gather more evidence.) If the appeals tribunal’s decision is also affirmative, you’ll get a letter back from the regional tribunal advising you that you’re now free to marry. If, at any point, you get a negative decision (i.e. marriage valid) or you don’t agree with the affirmative decision, you can choose to appeal the case yourself.
In most cases, there’s no formal “hearing” that you have to actually attend. There is a formal trial, but it’s not a courtroom proceeding. The process proceeds through paperwork - the tribunal will mail you letters requesting information, and advising you of any new developments. The only thing you would have to actually attend is the interview to give your evidence. If you’re concerned about the possibility of a hearing, you may wish to consult your case instructor at the tribunal and ask about specifics, but from what I understand this is atypical.
