The last time we saw...
There are three sets of caps on this page, from three different shots. In the first one, we have to move all the cropping to the left again, because Frodo's responding to another shadow on the wall to our right.
A few frames of a nice reaction shot, even though Frolijah's not in them:
This shot is very important in my "What is Frodo thinking?" theory for the scene. Remember that the last time Frodo saw Merry and Pippin, they were being chased by a horde of orcs! Their chances of survival didn't seem too high, and he hasn't heard anything about them since. In frame 18/19, when Frodo starts realizing who it is he's seeing, his expression changes from happiness to another round of being caught "between bewilderment and great joy."
[So what's a hobbit to think?
- First he sees Gandalf, who he knows is dead.
- Then he sees Merry and Pippin, who he assumes are dead.
I have to believe that at this point Frodo is beginning to realize that he must be dead, too.]
[Remember this expression - We'll be seeing it again a few shots later as the theory plays out.]