I'm not entirely certain to be honest, but it should be mentioned that Mueller is there under subpoena, which means he's legally obligated to answer all questions put to him, with few exceptions. Mueller could have had this another way, which was to come to an agreement with the committee limiting the scope of his testimony, in which case he would appear voluntarily. In that case, the agreement would limit the questioning, but also, without a subpoena, he technically could refuse to answer any question he didn't want to answer, or leave whenever he pleased. He could have had it that way. Instead, he chose to not accept the committee's overtures toward such an agreement, even though they said at the outset they would subpoena him if they couldn't come to an agreement. If he's desperate to not talk about certain things, I find that odd.
Sure, when your typical political type testifies before one of these committees, they tap dance, don't really answer questions, and act in the generally dishonest way that such people tend to act. And there's rarely any consequence to it. But Mueller seems like a rule of law sort of guy. Which suggests he may not try to be evasive or invoke meritless legal privileges. In short, I think it's possible he may be more forthcoming than most people expect, albeit in a low key sort of way.
I think the questioners need to treat it like a deposition and be thorough. He's not going to volunteer any information. I think he will provide what he has, but they're going to need to extract it from him piece by piece.
My guess is that Mueller wants his legacy in these events to read: he followed the law and did his job. Yet I can't help but think that anyone who looked into Trump for that long can't possibly have a high opinion of the man. So does answering a direct factual question put to you by Congress while you're there under subpoena make him seem political just because it hurts Trump? It doesn't to me, and perhaps it won't the Mueller either.