Though I wasn't bugged about the orcs running from the Balrog, I would say that it doesn't exactly follow the books. My interpretation is that the Balrog is the commander, and so pushes the Orcs and troll forward to fight the Fellowship. It's only after they fail does he enter the fray.
Don't have the book, but listening to the CD, I remember that the troll tries to enter the chamber door first, gets a sword blow from Boromir (which is ineffective), gets stuck by a Sting-wielding Frodo (would have loved to have seen that) and so withdraws. The orcs enter next and are overwhelmed by the defense. The orc chieftain attacks Frodo and is cut down by Aragorn. The FotR makes to leave out the back door when Gandalf perceives the 'other.' He stays and attempts to shut the door, but the Balrog comes into the chamber and starts a counter spell. Gandalf is strained, and so has to use a word of command, which destroys the door and cuts off the pursuit. Momentarily he gets a glimpse of some dark presence, but does not know what it is at that time.
In the movie, when Gandalf sternly tells Aragorn that "swords are no more use here," they are positioned correctly (at the top of a stair) but Gandalf has no door to shut, nor does it seem that the Balrog tries to come through that way. Too big, perhaps? Some action, I assume, from the Balrog starts busting up the place, and this actually helps Aragorn and Frodo get over the gap in the stair. No action by Gandalf to cut off the pursuit. Was it a timing issue, or did PJ not want to show Gandalf as that able?
Anyway...similar, yet different
.
Another point: I thought that Gimli standing on Balin's tomb was disrespectful, but the book states that Gandalf uses it as a table when he reads the account of the dwarf colony, and so it's not treated
that respectfully. Plus, Gimli chops the legs of an orc who stands on Balin's tomb, and again I think that PJ only wanted to place Gimli where he could be seen when he delivered his lines, as it's a stupid place from which to fight.