It's going to hostile and I believe lots of players will fight with each other, the ref will be happy to give red cards out.
I expect Real to field a really strong team and be very hostile to Barcelona. My scoreline madrid 2 Barca 1.
I expect this match to be heavily one-sided with Barcelona dominating the ball posesion, and Real Madrid just waiting for the counter attack. There are rumours about Zidane rotating the squad to rest some players like Modric, CR7, injured Varane and Carvajal, etc. Btw, this is going to be Iniesta's last Clásico, so I expect to see a massive performance from him.