You need to know enough, and only enough, to determine whether or not you will forgive the affair. Even though curiosity or outrage may be motivating you to uncover every single fact, it is not necessary to do so. There are many details you will eventually regret knowing, despite the immediate urge to learn everything.
If the affair is a picture, you need to see its frame (the boundaries of the affair: when it started, when it ended, how far it went) along with enough detail to identify it in a quick glance. You do not need a level of detail gained by examining every specific aspect of the affair. Do not be consumed by the need to see it all. You need information that is adequate, not exhaustive. You need enough detail to allow you to move toward forgiveness and healing, while avoiding specifics that could become unnecessary reminders of the affair in the future.
Remember, you cannot unhear an answer once it is given. I usually discourage questions that link the affair memory to specific places, events, behaviors, or dates since these tend to become constant reminders in years ahead. Avoid details that will allow the affair to sink its hooks any deeper into your consciousness, especially specific details about sexual behaviors. Don't give the affair that kind of power. You can always choose to ask more questions later on, if you need to.
I have never had a spouse come to me after recovery and say, "I sure wish I'd asked for more details" but I've had quite a few express regret over prying too deeply.
In addition to the caution regarding detailed inquiry, you should exercise patience in asking the WHY questions. Even though you may feel a strong need to understand all the reasons for the affair, you are unlikely to get any satisfactory answers to “Why did you do this?” if the affair was recently uncovered. There are a couple reasons for this:
Your spouse/partner has very limited self-insight into all the motives that have been at work. Since insights are limited, they are not able to give adequate responses to “why” questions even if they want to.
Issues in your marriage may have contributed to affair vulnerability. This does not mean you are to blame or that your spouse had an excuse, but these issues may be significant in explaining some of the “why” questions. However, it is difficult to have this conversations, especially early in the healing process, without feeling like you are being blamed. Aware of that risk, your partner may not be ready to discuss these things now, and you may not be ready to hear them.
Turn your attention to other types of questions (who, when, what, etc.) early in the recovery process and wait until your spouse has had time for deeper reflection, preferably with the help of a counselor, before asking why.