the lord of the rings - Why was Gandalf afraid of the Balrog of Morgoth?

As depicted in the film, Gandalf appeared to be shaken and in fear of the Balrog of Morgoth, even just as Saruman mentioned it. Why was this so? Did they have any previous encounters?

Answer

Why would Gandalf be afraid of the Balrog? Well, it's a Balrog. It's a fearful thing. :)

More seriously, though, the Balrogs were terrifying beings, even for Gandalf and others of his level of power. The Balrogs are Maia as well, just as Gandalf and the rest of the Istari are. See here from the Valaquenta, the second book of the Silmarillion:

For of the Maiar many were drawn to his splendour in the days of his greatness, and remained in that allegiance down into his darkness; and others he corrupted afterwards to his service with lies and treacherous gifts. Dreadful among these spirits were the Valaraukar, the scourges of fire that in Middle-earth were called the Balrogs, demons of terror.

Not only are Balrogs on the same scale as Gandalf, they were beings of terror and fire to begin with, while Gandalf, for all his power, wasn't a warrior. He is described as "Wisest of the Maiar", and as a servant of Manwë, and of Nienna, and of Lórien, the Valar of wisdom and judgement and mercy and dreams, and not of Tulkas or Oromë, the more warlike Valar.

Additionally, you have to remember that in Tolkien's cosmology, older is always better. The world is always in a state of decay. The wonders of the Valar when the world was new could never be exceeded even by the greatest works of the elves in the First Age. Even Feanor's Silmarils were but an echo of the light of the trees. Likewise, the hands of Man of the Third Age could never produce works as fine as the Smiths of Eregion in the Second Age, and certainly not of the works of Feanor himself.

So keeping that in mind, we remember that the Balrogs were the Captains of Morgoth during the wars of the First Age. It took all the might of Ecthelion, Lord of the Fountain and one of the greatest captains of Gondolin and elvendom to defeat Gothmog, lord of the Balrogs, and he took him with him to his death.

So when Gandalf heard that a Balrog still lurks in the depth of Moria, of course he would be afraid. A Balrog is a terror of the First Age. And something that was a threat in the First Age would be an unstoppable terror in the Third.