I know plenty of actors go really deep when they're preparing for a role (Daniel Day-Lewis famously wandered around the streets of DC as if he were Abraham Lincoln). But generally they drop the character after filming is complete. Which is why I think it's so cool that McKellen can still deduce like Sherlock (his film 'Mr. Holmes' had its festival premiere on February 7th but is being released in cinemas tomorrow on June 19th).
You can see the full video here. Sir Ian produces deductions about the interviewer and uses them to extrapolate details about his habits and personality. I guess our favorite detective's skills aren't that far-fetched after all!