When you're around someone for long enough, you can pick up mannerisms and craft suitable impressions pretty easily.
The longer the years, the more the effects of this are compounded. So when you're Mark Hamill, it stands to reason that you can do a spot-on impression of Harrison Ford, simply because you've known one another for close to 50 years.
Take a look.
He's right, too - if Harrison Ford wasn't so lazy, he'd make a great director. But yes, to go back to our original point, that is a pitch-perfect Harrison Ford impression and he even gets the shifty looks he does as well.
All he needs now is that slightly-odd looking earring to complete the look and he could make a decent living as a Harrison Ford impressionist.