You shouldn't use two films - you want to show how the concept of truth is explored in a variety of ways. Try a newspaper article.
'Film' in and of itself does not represent truth. It's the same as any other text - the composer is conveying something, constucting some meaning around the theme. It doesn't matter so much what the character says or does in isolation - what matters is the whole context. So, I don't know 'Shattered Glass' but if the character is caught out, what is the composer trying to communicate:
-he's naughty (not likely)
-there are some goodies (less likely)
-the pressure for a 'story' has superceeded the quest for truth
-society has unwittingly created a situation where the 'truth' is never sexy enough to sell
-the demand we have created for senstationalism has created a situation where we are unlikely to know the truth, or bare, unadorned facts anyway.
I don't know (cause I don't know the film) but whatever you decide you need to look at the overall message, ideas etc and then pick a scene, sequence and use it to back up what you have said. Filmic techniques are so broad - lighting, dialogue (although this is often secondary to the visual stuff), music, camera angle but its the why? what effect does it create? Do we look down upon a character so he seems intimidated, vulnerable - do we pan around a room to see chaos, do we suddenly cut to a close up that conveys a response to something???
good luck
