The 'Twilight Saga: Breaking Dawn - Part 2' actor says it's very easy for him to stay grounded despite all his success because he doesn't have enough confidence to get arrogant.
He said: "I'm quite an insecure person, so it's not hard for me to stay grounded. You have to be pretty dumb to let your head get big, especially when everything about your life is reported. Everyone knows you're just a vain moron."
Robert - who has rekindled his romance with 'Twilight' co-star Kristen Stewart after she cheated on him with director Rupert Sanders - also admits he doesn't really know what he's doing when it comes to filming movies and just hopes for the best.
He told Britain's Cosmopolitan magazine: "I don't really know what I'm doing when I'm filming, so a lot of the time it's like a tossing a coin if something comes out good or not.
"I can walk on to a set and have done all my preparation, but have no idea what's going to happen until I open my mouth."