Most people cooperate and have the same results. I’m sure a couple make it hard due to their rights but most that dont cooperate turn out to be criminal. Doesn’t matter if it’s a piece of gum or a trunk full of dope and guns.
Cooperation works both ways. If the ATF calls and the guy isn't a person of interest, they should explain why they want to talk to the guy. If the ATF is evasive or too general as to what they want to talk about, it's reasonable to believe they are looking at the guy as a potential criminal based on the information they have and want to throw some surprise questions at him.