It's called acting, darling!
Sir Michael Caine recalled his big break in Zulu (1964) during an interview with the Daily Fail. Fortunately, it was excerpted by Deadline, so I could read about it without risk to my IQ.
“They were looking for a cockney corporal and they thought I’d be great, but when I got there for the audition the director said, ‘Sorry, Michael, it’s been cast.’ I was used to being rejected, but it was a long walk to the door, let me tell you. “’Can you do posh?’ he asked me on my way out. ‘Of course I can do posh!’ So, I got the job playing a posh officer and it made my career.
Sir Michael Caine, via Caroline Frost, Michael Caine: “We Never Had Intimacy Coordinators In My Day, Thank God”, Deadline
Of course, this was back in the days when actors could pretend to be something that they weren't. There's no way he could get away with that now, unless he was Idris Elba.