How to learn and apply frameworks?

To answer your questions:

• "in depth study of the frameworks is necessary at all?" This depends on your own personal understanding and comfort with the underlying concepts of these frameworks. Please remember that frameworks are "organized" collections of the basic business concepts that you should know in order to solve a case. So, for instance, if you are not comfortable with the basic cost structure of a company, what belies fixed and variable costs, what does it mean when a company is more fixed cost or variable cost oriented... then it is best to dig deeper into that concept.
• "Is it just enough to read through them and try to gain a high level conceputal understanding and work from there?" It depends on the impression that you want to convey during the case interview. Remember: case cracking is about being consistent and organized in your thinking process and frameworks offer a first step in that direction. Reading through them won't be enough (especially if you're aiming for MBBs) - you need to at least be extremely comfortable with their basic structure and the frequent underlying questions that you need to ask.
• "how can I apply the framework during the interview without making it too obvious/seem like it was just memorized?" There are cases that are screaming "APPLY FRAMEWORK X" to be solved. Whereas others require a combination of two or more frameworks to be solved. The pitfall here is to blindly apply a given framework to all the cases that you can encounter. So, how can you apply frameworks without seeming having memorized them:
  • Don't announce that you're going to be using a given framework - announce a structure instead
  • Be comfortable about the concepts and questions underlying the framework so that you know which concept to apply/question to ask in the case (this ties in with your two previous questions)
  • Unless the case in question is extremely straightforward and obvious, rarely will you need ONE framework to solve a case - so as you go through the case, your initial structure (which is based off the initial framework you've used) will change as you're given data (and that change is brought upon by the other frameworks that you know).

Hope that helped.