Ghosts, huh. Jack's immediate reaction would've been there's no such thing, but he'd seen a few ghosts before, hadn't he? Not the flapping-sheets-in-chains type, but the Doctor could always mean one of those. "I'm in if you tell me what you're planning... guessing that it takes more than running around with a giant net."
The abrupt change of topic did catch him off-guard, but he quietly filtered the information away and tried not to think about it. He'd started doing that with most things involving dying since the year that never was. What he did say was "Okay. So you're not about to regenerate on me this time?"