To elaborate on that, Sigfried only joins if you bring him a "pure maiden," or a virgin. Since Sierra's been around so long, and especially seeing the way she acts towards guys... yeah, it's implied she's most definitely not "pure" at all.
Sierra's blatant attempts to hit on Klaus, and his utterly flustered reactions.
Nanami's impression of Hilda while trying to enter Muse City. It also comes with a Noblewoman's Laugh.
Several of Richmond's reports qualify. Notable examples are Siegfried and Zamza.
When you recruit Simone, you must have a Rose Brooch. He will offer to do anything to get it back. You can let him simply join, or you can order him to bark like a dog to get it back. Warning: he will NOT join your party if you choose the latter option, but by all means save before recruiting him. He will get down on all fours and crawl around barking like a dog. After awhile he will beg to stop, but you can force him to keep going.
Recruiting Sid and seeing him terrorizing Chaco. Up to this point Chaco had been an unnecessary Jerk Ass to the hero, which included stealing all his money upon his first arrrival in Two River. So seeing Chaco screaming and begging in fear feels rather satisfying after everything he put you through.
If you bring Nanami with you to the Highland border you entered with Rina, she will attempt to do the same thing that Rina did. Of course, once she drags the guy off alone, she jumps him and fails.
If you recruit both Gengen and Gabocha and they're not in your party, sometimes if you go up to the room in your castle where all the performances happen, they'll do a dance.