I wouldn’t be surprised, especially for A-listers, especially on-set. Eye-contact promotes conversations which, even when they are more stimulating than “Oh wow! You’re _____!! I loved you in that thing”, eat up time in a very busy production schedule. It’s even worse if the star is genuinely nice and personable, and sincerely appreciates their fans. It adds up to hours gone every day in 3-6 minute increments.