Open Sesame: The Safe Cracker - Hearts of Stone
This quest begins during the Open Sesame Hearts of Stone Story Quest and you must complete this quest in order to complete the Open Sesame quest. For this quest you'll be offered two different choices/approaches of how to get into the vault; you can choose a safecracker (Quinto) or the demolitions expert (Casimir).
Depending on who you choose during this quest, the results of the Open Sesame Quest will be slightly different, as will this quest. Below I have included a list of the differences you should expect during the Open Sesame Quest depending on your choice here.
Choosing Quinto: Able to avoid all the guards if you choose the right dialogue, will always fall into pit of Arachnomorphs due to the trap door, Quinto will side with Horst
Choosing Casimir: Always fight guards no matter your dialogue choices, trap door is broken due to explosion and Cassimir sides with Ewald
Aside from the dialogue and some other minor aspects of the quest, what's listed above are the only major changes you'll experience depending on who you choose. As aforementioned this quest changes as well depending on who you choose, so pick either of the options below to continue the quest.
Recruiting Casimir
Recruiting Casimir is much harder than recruiting Quinto - if you fail to answer correctly with Casimir or don't use Axii on him then he'll blow himself up. To get him to come down from the top of his house you'll want to tell him he is pathetic, it may seem counter-intuitive but you give him a target for his rage other than himself.
Casimir will come down from his roof and fist fight you, after you beat him you'll tell him about the job you have planned and he will agree to join you.
Recruiting Quinto
To recruit Quinto you'll need to go to a small camp southeast of the Herbalist Hut on your map. Here you'll find Quinto as well as a man you may have already dealt with before by the name of Hans of Cidaris. If you've completed the Contract: Creature of Oxenfurt Forest Quest then you'll have to pay Hans double the usual price to free Quinto (which will be 400 Crowns instead of 200).
You have a few different options though if you don't want to pay. You can haggle the price down, play a game of Gwent instead or taunt Hans and beat him in a fist fight. Beating Hans in a fist fight means he'll waive the price entirely... Losing a fist fight to him though means that he'll double the price yet again.
Tip: Each time the price increases the EXP reward will increase as well. If you'd like to earn as much EXP as possible from this quest, purposefully lose a fistfight to Hans to pay more Crowns.
If you refuse to tell Quinto about the job when you free him from the cell then you'll have to meet him at the Academy Gate in southern Oxenfurt at midnight in order to recruit him.