Witness is fairly straightforth. I'll give you some pointers about it. Techniques you must talk about: cinematography, costume, setting and symbolism..there are more....just make sure you use at least 3. Themes: good cop/bad cop, corruption, Amish World vs the Western World, Violence.
Gun is symbolic of the differences in the world. Amish - peaceful (samuel and rachel). Western world - violent. (john book)
Barn raising scene - traditional values of the Amish world (Book comes to accept the differences)
Humour - milking the cows vs coffee joke.
Music - frightening, slow and stopping during the murder. Up and down depending on the action.
That's about all I can think of off the top of my head....
