[beacon294]

I've been reading about expertise and deliberate/purposeful practice, starting with "Peak" by Ericcson.

Experts, as a rule, augment their practice with a coach, and they never stop doing that. The quintessential example is the olympian.

So, that is how they increase their practice surface area.

I think interest is also generally relevant, but it's not the core in established fields: coaching is.

I am interested in unestablished fields too, which may be fundamentally interest driven. Although IMO that interest may be more about establishing frontiers than the specific topic.