#2 Barcelona's width & midfield movement
With Suarez, Messi and Griezmann largely occupying central roles as a narrow front three, the full-backs are expected to provide the width, which is more or less true on both flanks for Barcelona with a few caveats.
Junior Firpo was the more attacking full-back of the two. Nelson Semedo made plenty of runs forward but didn't do so as aggressively as Firpo. Although, it seemed a part of Valverde's plan.
Sergi Roberto playing in midfield would advance to the wide right position when Barcelona were in possession, leaving Sergio Busquets as the lone defensive midfielder as Arthur advanced centrally or chose to hang back depending on the situation.
Roberto's movement allowed the likes of Messi and Arthur plenty of space in central areas where they do their best work and it worked a treat for most of the first half.