It's a linear one hero on a single journey film. At first sight, I thought it didn’t seem to have a fight back in the third act. But on second thoughts, the pursuit of Brian, and the involvement with Brian and his problems was a form of fighting back. The search for Brian became a quest.
Often the best way to see the function of a section of a film is to ask what would be lost if it were removed. Remove the quest to find Brian and the impact on his family when he’s found, its ramifications and the film would have been circling around the conflict between the father and his daughters.
It would have become all relationship line. It would have slowed right down. The revelation about Brian and the search for Brian not only inserted a powerful emotional charge, forcing the George Clooney character into emotional turmoil and lifting the emotional stakes, it also inserted physical action and a goal to pursue.
The result for the audience was an increase in empathy and suspense.Clever.
By the way, was anyone else puzzled that it was described in publicity as a comedy-drama? Comedy?