It's my understanding that most tickets to the screening will be set aside for the invited guests. But, there will be a Red Carpet outside and many of the stars, including Robert, are expected to attend.
My best advice would be to contact the Festival directly and ask them for more specific information. I would guess that the stars will appear outside on the Red Carpet, but there will certainly be a huge crowd. Best to get there very early to secure a good spot. Again, contact the Festival for more info.