Being good at interviews is a totally different skill to being good at doing the tasks. The interview process will always favour those that have corporate type roles (i.e. the Stella and Chris types) as they inevitably have a lot of practice at it. If you do a lot of corporate style interviews, you just learn how to conduct yourself.
The key things in any interview are enthusiasm for the job, preparation (knowing exactly what you have to offer and knowing exactly what the business you're going to work for does), and positivity (never bad mouth any company or person you have worked with/for in the past).
The final thing that a lot of people don't know is that you're not expected to be able to answer every question at interview. It's ok to say you don't know if you don't (though obviously not for every question). The honesty is appreciated and you get bonus points for not letting it phase you.
Also, the harsh style of interviewing does exist at investment banks, which is why Stella and Chris were totally unphased by it.
