You're being very cryptic about your reasons for asking OP, but I'll bite!
I'm one, now a theatre director and a tutor at one of the conservatoires (name changed).
Your initial question is flawed, in my view, as you seem to be certain that this hypothetical person (you? Your child?) has "no natural talent". I'm not quite sure what you mean by this or how you can discern that?
For example, using your example of piano playing. You teach the child the skills and techniques until they can play a tune to a recognisable standard. At this stage, they can take that work and practice and add their own emotion, play with dynamics, expression, pace, etc and inject "soul" and it becomes emotive to listen to. Or they might just get stuck at the recognisable standard but lack any flair.
Either way you're not going to know if they'll bring that expression or flair or whatever you want to call it until they know how to bash out the tune.
Acting is exactly the same. There are indeed building blocks - some of which a pp has named. You would never just dive into Ophelia in the way you describe. Before that you must read the script, know it, analyse it in a huge and laborious amount of detail, answer questions about it, know what's happened before, after, in between scenes, off text, and what your character is saying about everything and what's been said about them, their objectives, their actioning and more. You're a detective finding answers. Then you can explore the character physically and use techniques like the ones that have been mentioned. And this is before any actual rehearsing of any scenes begins.
Also for a run of performances the work gets layered and complex - doing the same night after night 8 shows a week in front of different live audiences, but needing to find ways to keep it fresh. Also screen acting a totally different skill.
Acting isn't just something you turn up and do. Someone may be engaging or have charm, or be able to deliver a speech or a joke in a beguiling way, but can they maintain a role to this depth over a long run of a year, or internalise the characters emotions and remain so focussed that their face can communicate the subtlest of emotions through a camera even when the room is full of 50 crew members staring at them.
So once a person has the building blocks, (through training) that's when they start to reveal the kind of actor they are visible of being.