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."