Bad performance or cringeworthy performance aside, Kartik has organically earned this film because of his past hits. So he's not been handed this film on a platter. Obviously, he will get trolled less.
On another note, comparing Kartik to Ranbir is fallacious. Talent being the key factor setting them apart.
Why not compare Kartik to Arjun Kapoor? Both are almost equally bad, but the one who got the film because of his last name will get trolled more than the one who got it because of his own credibility. And credibility here does not refer to acting skills, but building connection with audience.
Kartik and Ayushmann both have worked hard to reach where they are. That cannot be denied.
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