The Redditors on /r/cambridge routinely tell American applicants that the whole "extra-curriculars" thing is only relevant if whatever you did demonstrates enthusiasm for or ability in your subject, both of which you will need to survive. Ability for the obvious reason, enthusiasm because you will get the shock of it no longer being effortless and meeting peers who are better at it than you.
For me, the "HR interview" was "what would you say to convince me of your enthusiasm for physics?" (as in, I was explicitly asked that very question) not "tell me about how your adventures pogo-sticking up the Khyber on your gap yah made you a well-rounded person" (and the "technical interview" asked you do actually do some physics).
As someone who couldn't have afforded a gap yah and would probably have been too frail to go on one, I'm pretty happy about that.
The thing that I'm told they will do is look at your school and weight things like GCSE results and A level results accordingly (as in, if you are at a bad school, they'll make allowances for that). https://www.theguardian.com/education/2012/jan/10/how-cambri... is interesting from that point of view.