Greg Heffley is a very impulsive and a brain-hatching character and plays as the main role in both the book and film of "The Diary Of A Wimpy Kid" itself. In fact, he can get very arrogant and jealous throughout some point in the series. He shows this arrogance when he mentions in the film "Diary of a Wimpy Kid" that Rowley Jefferson's favorite role model, Joshie is meant to be for eight years old whereas in in the book "Ugly Truth", he mentions that it was meant for six year olds.
He is also quite guillible as he had mistaken Rowley for Holly Hills in a love note towards the end of the book "The Last Truth". I tend to believe that in the book, he is very 'less-motivated' and is not the attention-grabber you expected to be as in both films "Diary Of a Wimpy Kid 1 & 2: Rodrick Rules". He has been very unlucky more likely in the book that the film and this is truly very clear. In the film, Rodrick Heffley and Greg Heffley turns out to be friends in the film in the middle and towards the ending of the book whereas in the book, they are not the kind of friends you expect brothers to be. Greg and Rodrick maybe has possibly been friends at the end of "Rodrick Rules"(book) when Greg helped Rodrick pass his Science Experiment and stated "I hope Rodrick is lucky to have a brother like me..".
In the film, Zachary Gordon's Greg Heffley is more of a childish character and yet unpopular at first, but turns out to be a class clown towards the both films. He also makes a very successful relationship with Holly Hills in "Rodrick Rules" as stated twice in the film. It is likely that Greg is more luckier in the film than the book but not as much. He is still companions with Rowley and Chirag Gupta in the films.
I think Greg Heffley is a very sensational and hard-working character in conclusion but lacks motivation.
Thank you for reading.