Interrogation is not as simple as what we see, where you sit down, chat a bit, scare the person, and it's over. It's a very exhausting job. At the very least, you must outlast the person you're interrogating, right. That's why over the years, this kind of work has been delegated to juniors, and Old Xing himself hasn't personally handled one in a long time.
But that doesn't mean he's gotten rusty.
In the end, the means of interrogation are those few types everyone uses because they work, whether it's in language or actions. The tricks of the trade are mostly similar in every precinct.
What really reflects a person's level is experience and intuition.
Take this moment for example, upon seeing this guy who definitely didn't look like a good guy, Old Xing's intuition told him... it would be best to let him go now, don't investigate, don't ask, having no involvement with him at all would be the best approach.
But just like what was said before.